23-year-old Lee Ji Hoon secretly trains to become an idol instead of studying for a job, but starts to wonder if he has what it takes to become famous.

Woo Seung makes an upsetting discovery by being in the wrong place at the wrong time, and then meets Ji Hoon under awkward circumstances.

Hyun Jae immediately finds his way into a hospital, and then gets a gigantic surprise.

As Hyun Jae starts to come to terms with his new reality, his presence throws Ji Hoon and Woo Seung's lives out of whack.

Unsure of what to do, Hyun Jae pretends to have amnesia, and looks for a place to stay.

On the day of their respective exams, Woo Seung and Ji Hoon stress out about doing their best.

The aftermath of examination day for all three of the rooftop tenants hits home.

Hyun-jae overhears a song he once had written in the past but is surprised to hear someone else is singing it. He later meets and confronts him while accompanying Woo-seung at her workplace. At the TV show, Boo-hee exhibits a hidden talent and reveals so much more. After filming, she drinks the night away with Gwang-jae and falls asleep on the road home, only to be found by a bypassing Hyun-jae.

Astounded to find Hyun-jae after 20 years, Gwang-jae attempts to send him back to his timeline while hiding him from the other members of the family. Meanwhile, Woo-seung mistakes MJ (Cha Eun-woo) for Ji-hoon and accidentally leaves her notebook to him. After discovering the bank that used to keep his accounts had gone bankrupt, Hyun-jae begins searching for the mysterious number that keeps on beeping on his now-defunct pager.

Distraught after being laid off from her part-time job, Woo-seung invites Hyun-jae, MC Drill, and Ji-hoon for drinks at the loft. The day after, Hyun-jae and Ji-hoon, accompanied by Woo-seung, decide to participate on a talent content at the mall. MJ later returns the notebook to Woo-seung.

Gwang-jae finds Hyun-jae with Ji-hoon at the talent show and reveals to him a long-harbored secret. Meanwhile, Soon-tae asks Boo-hee to make preparations for Gwang-jae's birthday, which also happens to be the day Hyun-jae had disappeared. In an effort to repay his debts to Woo-seung, Hyun-jae decides to cover for her part-time job as a mascot.

Ji-hoon spends the day sneaking out between the Star Punch building and the school in an effort to hide his idol training from Gwang-jae. Meanwhile, Hyun-jae gets himself trapped inside the family's apartment after switching Ji-hoon's present for Gwang-jae. Later that day, the whole family holds a memorial service for Hyun-jae in order to part with him and to focus on celebrating Gwang-jae's birthday.

Gwang-jae's birthday is ruined by everyone's discovery of Hyun-jae just as they are beginning to part with him, as well as his accidental finding of Ji-hoon's secrets. Despite claiming to be a lookalike of himself, Hyun-jae is being suspected as his real self by Soon-tae and Boo-hee. Ji-hoon's friends later persuade him to convince Gwang-jae to allow him to pursue his training, but decides to reveal to everyone about it, as well as his secret harboring of his friends at his apartment.

Gwang-jae rejects an offer for Boo-hee to star as a gimbap mascot in an advertisement, thinking it as an undignified promotional activity for her. Worried about the financial state of World Entertainment, he later reluctantly asks Young-jae for a loan at Star Punch, where Woo-seung is also applying for an internship. While researching on the internet, Hyun-jae becomes a member of an online forum discussing conspiracy theories about his disappearance, including the ones that are linked to Young-jae and Gwang-jae.

Woo-seung invites Hyun-jae and Ji-hoon to dinner. Ji-hoon refuses, but was later discovered by the two at the same restaurant he and his colleague Do Hye-ri (Bona) are eating at. The following day, Hyun-jae tries to find the cash he had previously stashed at the loft, but was blocked by an electronic lock Ji-hoon had set up on Woo-seung's room. He later applies for a part-time job working for Bo-hee at the bakeshop. Meanwhile, Woo-seung begins her first day working at Star Punch.

Hyun-jae forces himself to Woo-seung and Ji-hoon's weekend getaway to explore the city. Gwang-jae later gives him an additional part-time job as a manager for World Entertainment. Despite Gwang-jae reluctance, Boo-hee accepts the gimbap commercial job despite having a fever that day. After an encouraging dinner with Gwang-jae, Ji-hoon musters up the courage to do what he had really wanted in his life. Meanwhile, Hyun-jae was able unlock the door on Woo-seung's room to find his hidden stash, but decides to wait for her instead.

After overhearing Woo-seung reject Ji-hoon's love, a conflicted Hyun-jae contemplates on whether to process his feelings for her or help his son win her over. The next day, a group of stans upload a candid photo of Ji-hoon on social media and causes it go viral; while Hyun-jae discovers a surprise after opening his hidden stash in Woo-seung's room. At Star Punch, Woo-seung's presentation in front of MJ and Young-jaegoes awkward after an intervention from Hyun-jae and Ji-hoon.

Worried about Gwang-jae's financial troubles, Bo-hee goes to Star Punch and asks Young-jae for a loan, but receives an offer to transfer her management to him instead. Ji-hoon becomes a viral sensation on the internet, and becomes a target of obsessive fan calls. Star Punch also takes notice of Ji-hoon's sudden popularity, and offers him a position in the debut idol group. A pressured Woo-seung is obliged to participate in the intern hiking trip, which ends with her being stranded in the forest. Feeling awkward about asking Ji-hoon for help, she calls for Hyun-jae's aid instead. Thinking it would help his son win her over, Hyun-jae passes the task to him. Ji-hoon must decide on whether to accept the Star Punch's offer or to help Woo-seung.

After learning that Ji-hoon had left to accept the offer instead of helping Woo-seung, Hyun-jae rushes to the hiking trail to look for her. With Boo-hee transferring to Star Punch, Gwang-jae begins to look for job elsewhere, but was passed off in favor of more qualified applicants. Drinking his problems away throughout the night, he is stumbled upon by a guilt-ridden Hyun-jae, who had just learned of his personal sacrifices to the agency and to him. Meanwhile, Soon-tae discovers his granddaughter Mal-sook (Lee Han-soo) is having a relationship at a young age, and is having a difficulty adjusting to the situation.

Hye-ri asks MC Drill to let her visit Ji-hoon after learning about his fate at the training agency. Worried about Mal-sook being cheated upon, Soon-tae confronts his granddaughter's boyfriend. Woo-seung and Ji-hoon confront each other about the awkward state regarding their relationship. Worried for the World Entertainment's debts, Ji-hoon, and Gwang-jae, Hyun-jae decides to reveal himself to Young-jae and lays a claim for a share of Star Punch Entertainment.

Claiming his share of Young-jae's profits and blackmailing him about the ghostwritten songs, Hyun-jae is hired as an executive producer at Star Punch, surprising Woo-seung. The two spend their time at their newfound job together. After having a makeover, Ji-hoon's candid photos on social media goes viral, and he is referred by Hye-ri to a different agency.

While Gwang-jae and Ji-hoon spend the whole day fishing, Hyun-jae and Woo-seung are left alone at the loft. That evening, Woo-seung gets locked out of her room, while Hyun-jae catches a fever after getting himself caught in the rain.

Woo-seung nurses Hyun-jae back to health, bothering Ji-hoon, who is beginning to notice something suspicious about Hyun-jae's past. At Star Punch, Woo-seung accidentally deletes an entire master recording, forcing Hyun-jae to take over and recreate the track from scratch. That evening, Ji-hoon asks Hyun-jae to keep a distance from Woo-seung.

Compelled to give in to Ji-hoon's wishes, Hyun-jae tries to avoid Woo-seung as much as possible. He later secretly switches his 1993 composition notebook with the one hidden in Young-jae's vault and discovers a clue in unlocking a mystery of his past.

Gwang-jae reveals to Ji-hoon the truth about Hyun-jae. At Star Punch, MJ discovers that his catalog had been written by Hyun-jae and asks for his help in writing songs. Meanwhile, Woo-seung is unexpectedly visited by her former roommate and best friend, leaving Hyun-jae to step up and act as her boyfriend.

Hyun-jae offers Woo-seung to sing a demo track and professes his feelings for her. Ji-hoon discovers that MC Drill was laid off at the agency and now works at a shipment warehouse. Feeling guilty of living off from Ji-hoon, Woo-seung decides to move out of the loft and into her own apartment.

Woo-seung overhears Ji-hoon telling Hyun-jae the truth about him. Worried about the uncertainty of the future of their relationship, she tries to avoid him. At World Entertainment, Gwang-jae persuades Ji-hoon and MC Drill to form together as a musical duo. Later that day, Hyun-jae takes a reluctant Ji-hoon to an overnight camping trip.

After cracking the code on his past, Hyun-jae discovers his long-lost money and uses it to terminate MJ's contract at Star Punch. He later persuades MJ to sign with Gwang-jae in order to secure an investor for World Entertainment. That evening, Hyun-jae later helps Bo-hee and Gwang-jae profess their love for one another. Feeling alienated in the present time, he contemplates on leaving it behind for the past.

Woo-seung prevents Hyun-jae from leaving her behind by making him spend the night on her apartment. The following day, Hyun-jae confronts Young-jae to confess about his disappearance and later uncovers the truth about it from someone else. After finding a way to return to the past, he becomes conflicted as staying with Woo-seung in the present would prevent Ji-hoon from being born. Meanwhile, MJ wants to reveal the truth about his songs to the public before Young-jae does.

Soo-tae's dementia impels him to forget his way home. Gwang-jae discovers what happened to money Hyun-jae left behind for them. Ji-hoon and MC Drill pay a visit to Woo-seung's apartment. Hyun-jae decides to return to the past to allow Ji-hoon to exist in the present.

Hyun-jae wakes up to find that he had traveled back to the past. But instead of returning to 1993, he finds out that he has journeyed to 1994. After seeking the help of a then-confused Gwang-jae to locate his past self, the present Hyun-jae persuades the depressed Hyun-jae of the past to prevent himself to commit suicide by telling him the tales of his future self and his son Ji-hoon. The past Hyun-jae then helps his future self to return to the future. Meanwhile in the present, Ji-hoon and MC Drill make their debut performance as a duo.

Woo-seung adjusts to a life without Hyun-jae. Despite losing MJ to World Enterainment, Young-jae retains an adversarial but cordial relationship with Gwang-jae, who marries Bo-hee. After a long absence, Hyun jae successfully returns to the present and decides to stay in it with Woo-seung while maintaining a friendly relationship with Ji-hoon. As Woo-seung, Ji-hoon, and MC Drill celebrate Hyun-jae's return, Ji-hoon envisions a bright future ahead of them, remarking that their "best hit" is yet to come.