If Hae-jun hadn't been curious, he wouldn't have stumbled upon a time machine. With a few button presses, he quickly figures out how to use it, and that marks the beginning of his journey as a time traveler. Yoon-young is occupied with her job as an editor for a renowned writer, leaving her little time to spend with her mother. Then, suddenly and unexpectedly, tragedy strikes as Yoon-young receives a call that her mother has been found dead.

Despite her confusion about the current situation, Yoon-young recognizes a small possibility of altering the future after a chance encounter with 19-year-old Soon-ae, prompting her to follow her closely. Soon afterward, Yoon-young learns about the mistreatment that Soon-ae is facing from the other kids and also notices that some of the children resemble those she knew from the future.

Yoon-young was able to enter school thanks to Hae-jun's help. As soon as she began her classes, she realized that one of her classmates was the future Ko Mi-sook. However, she was unaware of the danger that Ko Mi-sook posed, as she was extremely clever. Hae-jun ensured that Yoon-young remained at school until he could pick her up, but when he returned, Yoon-young was nowhere to be found.

With the day drawing closer, Hae-jun experiences mounting anxiety over the possibility of failing to prevent the murder, given his limited information and unexpected developments. While the school is on a field trip, a new student teacher is introduced, whom Yoon-young finds strangely familiar. Later that night, while alone at home, Yoon-young recalls the true identity of the student teacher.

Now that Yoong-young has developed trust in Hae-jun, she believes it's crucial to disclose her relationship with Soon-ae and Hee-seob to him. With the missing pieces of the puzzle given to him, Hae-jun finally comprehends the reasons behind Yoon-young's fixation on Soon-ae. More importantly, Hae-jun proposes an alternative perspective on what could have occurred to Yoon-young's mother, leaving Yoon-young stunned at her own foolishness for losing hope.

Hae-jun feels it's unnecessary to share what he saw with Yoon-young, complicating the situation. He grows agitated as events deviate from his expectations. Meanwhile, Yoon-young is surprised by Soon-ae falling for Hee-seob despite her attempts to break them apart. Later, Yoon-young understands why Hae-jun hid things from her, and as she talks, something clicks in Hae-jun's mind.

Hae-jun and Yoon-young feel relieved, knowing that Lee Ju-young is out of harm's way. Immediately, their attention shifts toward preventing a second tragedy. They hold on to the belief that as long as they remain vigilant and closely monitor the potential victim, nothing untoward will occur. However, their decision to solely focus on a single target may have been an oversight, as unexpected events tend to unfold precisely when and where one least anticipates them.

Hae-jun and Yoon-young find themselves in a dire situation as the events they thought they had escaped become a reality before their eyes. Yoon-young, who had hoped to change the future for the better, is devastated by the gravity of her circumstances. Setting aside the past, they now seek their next steps.

The situation looks grim for Hee-seob and Yoo-seob, but Hae-jun realizes the lengths to which Hee-seob is willing to go to safeguard his loved ones. Consumed by guilt over her sister's demise, Soon-ae takes it upon herself to aid Hee-seob in proving his innocence, but her father disagrees. Eventually, Yoo-seob discovers that Hee-seob has been apprehended. Hae-jun devises a solution to free Hee-seob from trouble, while Yoon-young anxiously waits for him outside an inn.

Hae-jun and Yoon-young have gained a deep understanding of how a minor mistake can have devastating and irreversible consequences for others, particularly when it involves family matters. They now direct their attention to Hae-kyung, whom they fear might be the next target. Hae-jun empathizes with Hae-kyung's situation and endeavors to prevent her from fleeing her home. Meanwhile, a thought occurs to Hae-jun, sparking his curiosity to personally investigate something.

In Hae-jun's memories, there is a notable absence of shared moments with his mother. Nevertheless, his curiosity drives him to uncover the truth about his mother's character. Writing holds paramount significance in Soon-ae's life, but her hard work hangs in the balance as the threat of Mi-sook stealing it looms closer. Yoon-young refuses to allow her mother to endure the same life that awaits her if she gives everything up.

The case grows more perplexing and convoluted with every investigation conducted by Hae-jun and Yoon-young. In the midst of her own emotional turmoil following her sister's demise, Soon-ae's predominant worry revolves around her mother's well-being. On a calm evening, when everything appears serene, Hae-jun receives an urgent and terrified call from Bum-ryong.

Hae-jun is caught in a difficult situation when he is discovered at the scene of a murder, coincidentally at the wrong time. The key to proving his innocence lies in revealing his existence as a time traveler. By demonstrating his ability to foresee the future, Hae-jun successfully vindicates himself and gains allies who assist him in his search for the perpetrator. In the meantime, Hae-jun tries to maintain a facade of disinterest toward his mother, but finds himself irresistibly drawn to Chung-ah.

Rather than idly waiting for the case to resolve itself or relying solely on the police, Hae-jun takes action by involving the media to expedite the investigation. The increased public attention creates unease among certain individuals, and Chung-ah is one of them. She unexpectedly opts to shut down her teahouse. As she prepares to leave the town, Chung-ah turns to Hae-jun and asks him to accompany her to the station, and the two share a little conversation.

After enduring a series of harrowing experiences, Hae-jun finds himself confronted by the culprit, whose identity leaves him utterly astounded. Remarkably meticulous, the culprit erased every possible piece of evidence before being caught. As Hae-jun starts to piece together the puzzle, the trials he faced begin to make sense. He meticulously follows the trail, step by step, in the hopes that it will lead him to the vital evidence he needs.

With everything seemingly restored, Hae-jun realizes he is unable to go back. Suspecting there must be a reason behind this, he embarks on a final set of visits to say goodbye to his close family and friends. Yoon-young follows a similar path, ensuring her loved ones are prepared for the unexpected future. However, during their last journey together, Hae-jun encounters a special someone whom he is unwilling to reveal to Yoon-young.