The year 2006. Two brothers, Nanba Mutta (age 12) and Hibito (age 9), happen to spot a UFO headed for the moon. The experience inspires them to make a certain promise.

With his mother's assistance and Hibito's encouragement, Mutta has been accepted for the New Astronaut Selection Exam. However, unsure of his next step, he visits Sharon, who works at a nearby observatory. After receiving advice from Sharon, Mutta is finally able to make up his mind.

After advancing to the second exam, Mutta realizes that the other applicants are much better prepared than he is. Desperate to prove himself, he stakes everything on the lung capacity test, as he has confidence in his trumpet-trained lungs. However, as the other applicants drop out, he finds himself in a heated battle with Itou Serika. Who will come out on top?

After a long week, only the final interview remains in the second JAXA exam. Mutta is drawing attention as Nanba Hibito's older brother, but he insists he won't spend his entire life as Nanba Hibito's big brother.

After completing the second JAXA exam, Hibito invites Mutta for a free trip to Houston, Texas, through NASA's family support program. Upon arrival, he finds himself chased by a pug named Apo, and is then reunited with Hibito. However, Hibito expresses disappointment.

As Mutta arrives at the Johnson Space Center to observe Hibito's training, one of the JAXA judges brings up the reason Mutta was fired from his previous job. As Hibito appears on a television show that night, Mutta is surprised when Hibito starts praising him. Just then, Mutta receives a call from his former workmates informing them of what his former boss had told JAXA. As Mutta becomes convinced he has lost his chance of becoming an astronaut, one of the other JAXA judges, who had been observing Mutta and Hibito's constant visits to JAXA, meets up with Mutta's former workmates to get the whole story.

As Hibito notices Mutta acting strangely, he suspects that something is up. Mutta eventually confesses about his high chance of failure, and appears to have resided to think he has no chance. Meanwhile, Hoshika has trouble trying to convince another judge to change his mind about Mutta. As Mutta spends time with the pet walkers following being told off by Hibito, the restaurant he is visiting is attacked by a fire extinguisher weilding robber.

As Mutta's somehow manages to apprehend the robber by chance, he gains a lot of attention and is praised as a hero on a live TV broadcast, which is seen by Hibito and the JAXA judges. Afterwards, Mutta receives a video message from his parents, revealing he has passed the second exam. Before leaving for Japan, Mutta discovers a hidden set of wills written by Hibito which leaves him concerned.

As he arrives in Japan, Mutta remains concerned about the wills Hibito wrote before going outside to see the International Space Station in the sky. Later, he meets up with Kenji and Serika to celebrate making it to the third exam. The three talk about their motives for going into space, with Serika wanted to go on the ISS, despite the fact it will probably be shut down by the time she makes it to space. He hears from another collegue that she wants to carry on the dream of her late father to develop medicine to cure a disease, which can only be done on the ISS.

Before heading off to JAXA for the third exam, Mutta visits Sharon who tells him not to worry about not having a reason to go into space like Serika and Kenji has. As the exam begins, the applicants board a strange bus supposedly heading towards a top secret facility, with only Kenji noticing there are cameras onboard. An hour into the journey, they begin an exercise in which each applicant spends ten minutes talking to each other. Afterwards, they are asked to fill in a survey ranking everyone's potential to be an astronaut, followed by a meal and a rest before the bus arrives somewhere.

Arriving at an undisclosed location, the applicants are shown footage of the last moments of a group of astronauts before they died due to a chute failure upon reentry, in order to determine their willingness to continue with the exam. After everyone agrees to continue with the exam, the group is split up into three teams with each team spending two weeks together in a sealed environment before determining which two of them would make the best astronauts. Mutta is placed in a team with Serika, Reiji Nitta, Naoto Fukuda and Yasushi Furuya. The first assignment involves figuring out what the current time is, where Mutta comes up with a surprisingly precise answer.

Mutta comes up with a method for his prediction, hiding the fact he had simply noticed what time it was when he left the bus. As Serika's hunger keeps her up, she recalls when she asked her father what the meaning behind her name was. The next morning, as the teams have to decide how to handle the food supply, they also discuss how to go about deciding which two candidates to pick to be astronauts. After everyone on Team A introduces themselves, Serika decides to let Mutta call her by her first name.

Nitta grows concerned about a heart monitoring bracelet everyone is wearing, as well as a rule in which if one team member leaves, the entire team is disqualified. After a few physical and mental tasks, the teams are tasked with writing a letter to win over a journalist who is critical of space exploration. Mutta recalls when he and Hibito went to a lecture held by Soichi Noguchi about how seeing things from a different perspectives brings new solutions. After thinking about the story, Mutta decides the right action is not to protest but instead show her the third dimension offered by outer space, which will be made apparent once Hibito reaches the moon.

As Naoto recalls how his work in developed a manned spacecraft alienated him from his family, his glasses get broken when they are accidentally stepped on by Yasushi. Despite his reduced visibility, Naoto becomes determined to not let it stop him. After Yasushi hears from Serika about how Naoto quit his job in order to get a chance for astronaut selection, he swallows his pride and asks mission control to order new glasses for him.

As the JAXA staff decide to order Naoto new glasses as per Yasushi's request, Hoshika steps in to observe the exam. The next day, as Team B is put on edge when an alarm starts going off in the middle of the night, Team A discovers their clock has gone missing, later finding it smashed. The applicants are soon tasked with completing a blank puzzle within three hours. As the situation in Team A becomes tense as Yasushi accuses Serika of breaking the clock, Mutta tries to calm the situation by having everyone talk about space, getting them to consider JAXA broke the clock so they could learn to depend on their own sense of time whilst hiding the fact he saw Naoto break the clock.

After the puzzle challenge ends before anyone manages to complete it, Mutta confronts Naoto about what he saw, but he only tasks him with finding out the reason why he broke the clock. Meanwhile, Team B continues to be plagued by the mysterious alarm, which is secretly being controlled by one of the participants, and soon find their clock has been tampered with as well. Meanwhile, as Serika grows concerned that she's used too much food for cooking, with not much left for the remaining four days, Team A is soon given their own alarm problem.

As Team A searches for the mysterious alarm, Mutta notices some strange behaviour in Yasushi. Haven't been reminded about the cameras by Naoto, Mutta believes JAXA is behind these incidents. He recalls what he learnt whilst visiting Houston, in which green cards are given to astronauts in training instructing them to create problems. Thus, Mutta correctly deduces that both Naoto and Yasushi had been given green cards ordering them to break the clock and set off the alarm. Later, Serika confesses about the food shortage to Mutta, who mistakenly believes it to be another green card order.

After a few more green card related incidents, Mutta is given his own green card, prompting him to make random noises throughout the day. Meanwhile, as Team B continues facing issues, Kenji is put under pressure when he is told to withdraw from the selections, feeling he has done nothing to improve teamwork. As Kenji seriously contemplates withdrawing, he recalls what his daughter said to him, realising she was telling him to do his best and regaining his confidence.

As the exam approaches its final few days, Kenji gets to know his teammates better, which Hoshika musing that he didn't give Kenji a green card earlier in order to test his leadership skills. Meanwhile in Team A, as Mutta reveals they don't have any food left for tomorrow, he manages to gather some ingredients for making udon noodles. Remembering how he learnt it from Sharon along with Hibito, Mutta gets everyone to make udon together. Having not had anyone share his interest in space whilst he was in school, Mutta becomes pleasantly surprised when everyone else becomes surprised over how he knows Sharon, and they spend time talking about space together. The mood soon heavies when they are reminded they still need to choose two people to become astronauts.

As the last day of the exam arrives, the teams are told to do what they want for the rest of the day before deciding on thier selections, so Team A decides to clean up the place. As Team A struggles to decide on a method of choosing two astronauts, Mutta recalls the advise Sharon gave him and suggests they decide via a game of rock, paper, scissors so that everyone has a fair chance. As Yasushi accepts to this, he shows off his recordings of everyone's scores, revealing he's been in a last place. Mutta reveals that he got the idea to use rock, paper, scissors from Yasushi, and he agrees to go along with it.

After Team A decide their selections after one round, the third exam comes to its end and Team A is met with applause from the staff. Upon venturing out into the open sky again, the applicants discover they had been at JAXA the entire time. As Mutta and Keiji discuss how those who weren't selected may still have a chance, Yasushi, who lost out in the match, visits a much-admired professor, Hiroto Baba, who shows him his newly designed spacesuit. Whilst heading for a celebratory meal with the rest of Team A, Yasushi recalls how he got interested in space, but was troubled that he was too short to become an astronaut. This led him to admire Baba when he invented a smaller spacesuit which lowered the minimum height requirement to become an astronaut.

Team A get together to celebrate the winners of the match, Serika and Reiji, on passing the third exam. Meanwhile, the JAXA staff have discussions about which of the remaining candidates to select, with Hoshika pointing out how Team A was the only one who promised to get together upon leaving. Four days later, it is revealed Houston will select one male from the remaining candidates to become an astronaut, with Mutta assuming Kenji is likely to be chosen over him. As Mutta worries about his lack of luck, Hibito carries on his training, curious about what Mutta's reply will be. As Mutta hangs out with Naoto, it is revealed that Mutta had been chosen to go to Houston which boths excites Mutta but also causes him to be concerned about Kenji.

Mutta takes up a part-time job as a shopping mall Santa to earn some money before his trip to Houston. Meanwhile, Yuri Teshima, one of the candidates from Team B, shows his father his passion for extra terrestrial life. As Mutta hears from JAXA about when he is to depart for Houston, he learns that Kenji was in fact chosen alongside him, as Yuri had decided to turn down his application to pursue a career in marine biology. Sera visits her father's grave, leaving a diary of everything she got up to in the past year, whilst Mutta visits Kenji. With the final exam in two months, coinciding with Hibito's mission into space, Mutta decides to head to Houston ahead of schedule.

As Hibito trains to use a space vechile for his moon mission, Mutta does some lawn mowing for Ozzy. Later, Mutta has dinner with Jennifer and Lowry, Hibito's backup, where he learns the examiner for the final exams will be Takio Azuma, who allegedly holds a grudge against Hibito for being selected to go on the moon instead of him, putting Mutta at a disadvantage.

As Hibito recalls how he came to admire Azuma, he tells Mutta how his final astronaut selection exam was an interview with one of the Japanese astronauts. As Mutta ponders how he should spend his time before the exam, he witnesses Hibito piloting a jet craft, reminding Mutta of how he came to support him. As the day count down towards Hibito's launch, Mutta trains his body and mind with Ozzy to prepare for his interview. Seven days before the launch, as Hibito is moved into quarantine, Serika and the other candidates travel with Mutta's parents to Houston.

As the day of the final exam arrives, Nanba is hindered by aching leg muscles due to all the squats he did the other day. After entering the interview room and meeting the examiners, including Azuma and Soichi Noguchi, Mutta sits down only for his chair to break due to a combination of a screw loosened by one of the examiners and Mutta's tenseness in hiding his cramps. After the interviews end, the candidates have a celebratory meal with the examiners. Unbeknownst to the candidates, the interview was a decoy intended to drop their guard as the examiners make observations outside of an exam environment. As Mutta is secretly probed for his observations on the other candidates, he comes face to face with Azuma.

As Mutta decides to converse with Azuma, he asks him a question that another astronaut previously asked him; "Are you prepared to die?". Mutta gives an honest reply that he would try to survive if possible, which is similar to how both Azuma and Hibito answered the same question. As Mutta and other candidates head off to Florida, he recalls how he and Hibito spent months trying to get footage of the UFO they say, since noone else saw it due to the Zidane headbutt incident. Upon arriving in Florids, Mutta and his parents go to see Hibito, giving him a DVD of the footage they filmed together to take to the moon with him.

Hibito recalls how Mutta was bullied by those who didn't believe him about the UFO. Wanting to find proof of UFOs existing in order to stop the bullying without resorting to violence himself, he decided to make his own UFO, giving the footage he filmed to Mutta for his 13th birthday. Despite obviously telling it was a fake, Mutta appreciated the thought and treasured the DVD. When the bullies started turning on Hibito instead, Mutta fought against them with his headbutt.

The Mars V rocket containing parts for the construction of a moonbase is successfully launched into space. On the night before Hibito's launch, Mutta is visited by his mother, who expresses her concerns about the launch. On the morning of the launch, NASA reports a 60% chance of the launch being postponed due to the weather, but Hibito remains optimistic that the launch will go ahead. As Mutta and Kenji visit the news center for updates on the weather, a kid with a punk hairstyle sets Apo loose.

As Mutta and the others search for Apo, Mutta finds him being carried off by a strange old NASA employee named Deneil Young, who takes Mutta for a ride to a place featuring abandoned training facilities. Reaching the top of an old blockhouse, Mutta is treated to a spectacular view of the launchpad. As the Mars I finally lifts off, Mutta stands back and watches in awe.

As Hibito and his crew finally make it into the weightlessness of space, Mutta thanks back to when he and Hibito attended a broadcast of Akihiko Hoshide aboard the ISS, answering Hibito's question and giving him the encouragement to become an astronaut. Back in the present, Deneil reveals to Mutta that he was the one who taught Hibito to fly a jet, saying they will meet again on the airfrield should Mutta be selected.

As the candidates celebrate Hibito's launch, they are informed they will return to Japan and will hear of the results a week after Hibito lands on the moon, though Mutta opts to stay in Houston until Hibito lands on the moon. Later on, Hibito gives an interview aboard the Orion as it makes his journey to the moon, which is broadcast across Japan. As Mutta spends time with Jennifer, she shows him Azuma with his family. As Mutta goes to greet them, Azuma shakes his hand and invites him to join them, where he learns more about him from his wife. Later that night, Azuma recalls the pressures her faced from the press after orbiting the moon and how it was Brian Jay who reccomended Hibito be the first to walk on the moon.

As the Orion approaches the moon for landing, Hibito receives a video message from Mutta. The whole of Japan tunes in early in the morning to watch the live broadcast of the moon landing, which Hibito manages to pull off successfully. As Hibito prepares to take his first steps on the moon's surface, he recalls Mutta's video message, assuring him of Azuma's approval. With that in mind, Hibito turns his first step into what would come to be known as 'the moon jump', although Mutta is a little embarrassed by Hibito's 'inspiration first words'.

As the moon mission begins its progress, Hibito notices something shiny on the moon's surface before helping to ease the tension of the rest of the crew. Meanwhile, Mutta arrives back in Japan, bring Apo home with him, hoping he will bring him luck in passing the exam. Whilst grocery shopping, Mutta ends up winning a consolation prize in a lottery, believing he has used up all his luck and becoming paranoid. On the day of the selection announcement, Kenji is the first to be called by JAXA.

As Kenji receives his call from JAXA, he thinks back to how his life was before all of this. Though he was generally satisfied with his job, he had always wanted something more. He also recalls when he first met his wife, Yuki, who would later be the one who told him about the astronaut selection programme. This changed his daily life a little more as he began to study towards his dreams. As the results are finally announced, Kenji learns that he has passed the exam to the delight of his family.

As Serika visits her father's grave after hearing about her results, she recalls her childhood. Being a biologist who studied 'little universes', Rinpei Itou inspired Serika to want to be an astronaut. Rinpei soon became hospitalised with a paralyzing brain disorder, so he asked Serika to start a journal to tell him about her daily life. Rinpei's condition soon worsened, as his condition was revealed to be a rare disease. Reading up on Chiaki Mukai, who was an astronaut who developed cures aboard the ISS, Serika became determined to be the same so she could find a cure for Rinpei's condition. However, he passed away soon afterwards. Having learnt that she has passed her exams, Serika celebrates with a nostalgic ice pop.

As Mutta becomes anxious about whether he has enough luck to pass, Hoshika decides to share the load of revealing the results. The first person he calls is Mizoguchi, who is shocked to learn he has failed as he was always the top in everything he did. When asked why he failed, Hoshika explains it is his lack of depending on others that caused him to fail. Afterwards, Hoshika asks Mutta to meet him at the park, where he personally reveals that Mutta has passed his exam. Hoshika then takes Mutta straight to the press conference.

Upon arriving at the conference building, Mutta meets with some of the unsuccessful applicants before heading to the main hall, where he is greeted by Serika, Kenji and Reiji, along with Team B's Ena Kitamura. As Hoshika gives Hibito the news of Mutta's success, Mutta manages to get through the conference without messing up. As the gang get together following the conference, Naoto joins them in celebration whilst Yasushi sends Mutta various texts congratulation him, explaining how he was always rooting for him.

As Mutta and the others begin their first day at JAXA, they receive a talk from Murasaki about the training they will go through to become fully fledged astronauts. Meanwhile on the moon, Hibito and co-astronaut Damian are sent on a mission to recover a damaged space probe, struggling with the illusions the moon plays on perception, whilst fellow astronauts Buddy Waters and Karen Jones host a television special. Whilst trying to open a compartment on their buggy, Hibito and Damian lose track of where they are going and end up driving into a crater.

Hibito finds himself trapped inside the crater, seperated from Damian and unable to contact Houston. As Hibito searches for Damian, he manages to find the probe they were looking for. A flare alerts Hibito to Damian's position, where he finds Damian has injured his leg and his temperature regulation system is broken, meaning he will soon freeze to death. Although Damian tells Hibito to leave him behind, Hibito instead decides to use the battery from the buggy to power the probe in order to get him out of the crater. As Hibito attempts to climb up the craterside to reach the buggy, he loses his grip and falls, damaging his suit on impact.

As NASA discusses rescue operations for Hibito and Damian, Azuma suggests that they send remote controlled probes to assist them whilst the other astronauts wait for their buggy to be charged. Although Hibito recovers from his fall, Damian informs him that his main oxygen tank has been punctured, leaving him with just eighty minutes of oxygen remaining. Despite this, Hibito resumes his attempt to rescue Damian, managing to reach the probe and reactivate it. Meanwhile, Mutta, having just heard the news, arrives at JAXA and gives them some input based on how he feels Hibito would act, though this advice is ignored by NASA.

Hoshika tells Mutta that Houston will be able to tell if Hibito moved if he fires his remaining flare. Meanwhile, Hibito, who has less than twenty minutes of oxygen left, decides to use his last flare as a makeshift fireplace to provide Damian with some neccessary heat. Hibito eventually reaches the top of the crater and exposes Damian to the sunlight, but becomes stressed when he can't communicate with anyone. Meanwhile, Houston learns of their location, which was at the point Mutta predicted, and learn that Hibito has less than ten minutes of oxygen remaining. After making sure Damian is properly shaded, Hibito decides to take a walk on his own, preparing to walk into the ravine again so he can see the stars once more. Just then, he spots the flash he saw earlier and decides to investigate it, discovering it to be a small astronaut figure left behind by Brian.

Hibito reminisces about Brian, who was something of a mentor to him. Checking the back of the figure, Hibito finds a picture of a young Brian with his brother, realising Brian left the figure there so both brothers would eventually stand on the moon together. The oxygen reserves in Hibito's suit soon runs out, putting Hibito through pain as he gasps for oxygen before almost losing consciousness. Just then, the oxygen producing drone named after Brian arrives on the scene, allowing Hibito to obtain some oxygen in the very nick of time.

It is revealed that Hibito was able to be saved because of Azuma's plea to NASA to take into account Mutta's theory as the worst case scenario and send the Brian drone over to where he suggested. Meanwhile, Mutta is relieved to hear of Hibito's safety once news reaches JAXA. Whilst the other astronauts, Freddy Saturn and Buddy Walters, bring Hibito and Damian back to base, they reminisce about all the things Brian taught them. Three days later, Mutta gets a chance to talk with Hibito, who gives him his deepest thanks for helping to save him, promising that they'll both stand on the moon together.

Mutta pays a visit to Sharon, who is shocked to hear how much danger Hibito was in. She reveals that her astronomy team has received funding to build a lunar telescope on the moon, which would allow people to see further into space, an idea spurred by her late husband, Shinichi, who named an asteroid after her before he passed away from an illness. This spurs on Mutta to fulfil his promise to Sharon, along with his promise to Hibito. Later, Mutta and the others receive their official JAXA jumpsuits before being informed they will be travelling to Houston tomorrow to begin their joint astronaut training.

As the astronaut candidates arrive at the airport, Serika and Ena think back to their last meals with their friends and family. Serika had dinner with her friends and grandparents whilst Ena spent time with her younger sister, Kei, helping her decide what she wants to do in the future. Meanwhile, Mutta attracts a lot of attention after Murasaki tricks him into putting on his jumpsuit in the airport. Meanwhile at NASA, astronaut Vincent Bold is assigned as the training instructor for the new trainees. Upon arriving at Houston, Mutta meets up with Ozzy, who helps him and Kenji set up accomodation in Hibito's house.

Whilst Serika and Ena manage to find a place to live, Mutta becomes curious about something Murasaki allegedly mentioned to Kenji about the training during the flight. Deciding to ask Murasaki himself, he mentions how a few astronauts always seem to leave at some point during the training due to not being up to snuff. He goes on to explain the role of astronauts is akin to those of actors, stating he himself has never gotten the chance to 'stand on stage'. Later, the group go to the mission control center, where Mutta hears from Jennifer that Hibito probably won't be approved for any EVA operations for the duration of the mission. Afterwards, a party is held for all the trainee astronauts, where the group meet Amanti Patel from India, who has a hobby of reading people's fortunes. When asked to read Mutta's future, she has an unsteady feeling.

The astronaut candidates are briefed on their training, with Bold stating he plans to make everyone go through two years of training in a year and a half, to the surprise of his fellow instructor, Larry Bison. He starts off with the hard stuff right away by putting everyone through survival training in the desert, where they must walk all the way to Amarillo. Mutta's group, consisting of the JAXA candidates and Amanti, meet up with their advisors, Mike Davis and, to Mutta's surprise, Lowry, who provide them with lunch before informing them they'll need to forage further meals themselves.

As Mutta's team find their rations and set up camp for a night, they are informed they are currently last in the rankings, which requires one person beside the leader to take a punishment. Mutta volunteers for the punishment, in which he must stand at attention for a whole hour, but the group soon cheer up when they are treated to a beautiful view of the stars. Later on the fourth day, as Nitta is chosen as the leader, he appears to be a bit more strict with his desire to increase their ranking, whilst also expecting a call from a certain someone. Just then, Mutta pushes Nitta out of the way of their ration box when he notices a rattlesnake. Following the incident, Davis issues the leader with a handgun to defend themselves against dangerous wildlife. On the fifth day, as Mutta acts as leader and helps the group push onwards, Nitta suddenly tells the others that he must head back.

Nitta explains that he had left his phone behind somewhere, which he needs as he is expecting an important call. Nitta opts to go alone to search for it, but Mutta, being the leader, decides it would be best to search alongside him at night once they set up camp. Along the way, Nitta explains he is waiting for a call from his younger brother, Kazuya, who he hasn't spoken to in two years. Mutta suspects that Nitta may have dropped his phone near the previous food crate when Mutta tackled him, which motivates Nitta to rush to its location. Arriving at the crate, Nitta finds his phone as it flashes, only to find he had barely missed a call from Kazuya.

As Mutta and Nitta head back to camp, discussing their relationships with their respective brothers, Kazuya looks up information about the astronauts, finding a video interview with Nitta. In the video, Nitta explains how feelings that you may not belong in this world can help drive you to go beyond it, stating some of the things Kazuya taught him when they were younger. After returning to camp, Nitta explains to the others about his brother whilst Mutta once again worries about Amanti's prediction. As a starstorm flies above the desert, Kazuya starts to venture outside his own little world.