Season 1

Part 1: Yoon Tae Yi and Yoo Min Hyuk time travel from 2050 to 1992 and recover the "Book of Prophecy," a book that reveals the tragic end of time travel. However, Tae Yi finds out she is pregnant and decides to stay and have the baby by herself. She changes her name to Park Sun Young and gives birth to Jin Gyeom, a boy who grows up lacking pro-social emotions. Part 2: After celebrating his mother's birthday, Jin Gyeom finds his mother dying on the floor. Her death hits Jin Gyeom hard, and he decides to become a police officer. Now in 2020, Lieutenant Park Jin Gyeom takes on a case where a kidnapped child claims she was with her mother from the future.

Part 1: After chasing down a suspicious drone, Jin Gyeom thinks he sees his mom and suddenly gets hit by a car. He's not the only one who claims to have witnessed a drone, and strangely, reported sightings all occur at odd crime scenes. Jin Gyeom continues to work a kidnapping case involving a mom from the future, hoping it will lead to seeing his mother one more time. Part 2: A time traveller out to get revenge escorted by Alice, an organization providing time-travelling services, commits murder in the past. Jin Gyeom chases him down but loses him. When Alice tries to ban him from future time travels, he takes down an Alice guard and makes his way back to the past and knocks on Jin Gyeom's door.

Part 1: Agents from Alice try to recover the time card in Jin Gyeom's possession. But they don't realize he has two of them. Jin Gyeom, who thinks Tae Yi is his mother from the future, asks her to take a look at the bizarre card to see if she can figure it out. Meanwhile, Jin Gyeom looks into Tae Yi's family. Part 2: A desperate mother from the future takes drastic measures to stay with her child. Although she successfully retakes her past life, it puts the child's life in danger and exposes time travel. Meanwhile, the card Jin Gyeom gave to Tae Yi does something strange in front of her eyes and finally gets her attention.

Part 1: Captian Ko recovers in the hospital after getting stabbed by the desperate mother who gets away with her daughter. However, Alice agents and Jin Gyeom track her down. Sergeant Dong Ho manages to handcuff Alice agent Min Hyuk, but he disappears into thin air. Meanwhile, Tae Yi goes through Jin Gyeom’s late mother's possessions for additional clues and tells Jin Gyeom she was never just an ordinary housewife. Part 2: Tae Yi begs Jin Gyeom to let her continue investigating the card, but he won't let her even touch it. Jin Gyeom meets with Seok Oh Won, a director at a research facility, and suspects him to be his mother's murderer. Meanwhile, a drone appears outside Tae Yi window.

After the accident, Jin Gyeom finds himself talking to Tae Yi in the year 2010. He calls his mother and warns her not to leave the house, but it's already too late. As he witnesses her dying again, she sends him back to the present. Meanwhile, Alice takes note of a time travel glitch and plans to bring Jin Gyeom to Alice.

After meeting with Lee Se Hoon, Jin Gyeom finds a murder case file with a picture of Min Hyuk, an Alice agent, from 1992. Tae Yi triggers something on the time card causing Alice to respond and show up on the scene. However, Jin Gyeom gets alerted and saves her but loses the time card to Alice. Later, Min Hyuk returns after learning a copy of the time card's program got made.

An assassin from the future tries to kill Tae Yi, but Jin Gyeom once again saves her. The killer leaves a strange, indecipherable formula on Tae Yi's wall, leaving the police and Tae Yi baffled. However, his cellphone reveals a possible accomplice. Meanwhile, Seok Oh Won tells Jin Gyeom a little more about his mother and The Book of Prophecy.

To keep Tae Yi safe, Jin Gyeom and Tae Yi move into the house Jin Gyeom grew up in. The police focus on identifying the suspect who tried to kill Tae Yi and his accomplice. Back at Alice, Min Hyuk believes Lee Se Hoon might have the answers needed to find The Book of Prophecy, and Seok Oh Won remains missing. Later, Tae Yi finds an article by a mathematician, which uses the same approach in the equation left by the killer.

Tae Yi finds herself in 2021 desperately looking for Jin Gyeom, but he is not alive. With the time card still in her hand, she goes to Oh Seok Won and learns that Jin Gyeom died looking for the last page of The Book of Prophecy. She also finds out she was the last person to hold the missing page. Meanwhile, the body of an unregistered time traveler disappears, and Jin Gyeom discovers he can manipulate time.

Min Hyuk shows up and saves Tae Yi from time-traveling sickness, but he doesn't tell Jin Gyeom who he really is. Min Hyuk vows to find whoever killed Tae Yi so that he can reveal to Jin Gyeom that he is his father. Tae Yi recovers and determines to change Jin Gyeom's future, but no matter what she does, it's just as she remembered it. Meanwhile, Lee Se Hoon gets released from prison.

After he goes missing for quite some time, Seok Oh Won shows up alive. However, he doesn't have much to say about his captor, and only asks to be left alone. Meanwhile, Tae Yi confronts Jin Gyeom about the security footage containing Captain Ko, and her suspicions grow around him. Min Hyuk from Alice visits once again to protect Jin Gyeom.

Captain Ko tries to fulfill his promise with Jin Gyeom but ends up losing his life. A different Seok Oh Won, one who wants to keep time travel possible, takes one step ahead of Jin Gyeom. However, now that Jin Gyeom and Tae Yi know about the last page of The Book of Prophecy, they seek it to avenge Captain Ko's death and to stop time travel.