In 1993, Yoo Hyun-jae, a member of the famous rookie duo "J2," suddenly disappears. Fast forward to 2017. Lee Ji-hoon, a student at Seoul National University, keeps a secret of wanting to be an idol from his foster father, Lee Gwang-jae, the president of World Planning Entertainment. Ji-hoon lives with Choi Woo-seung and MC Drill, and Yoo Hyun-jae, who disappeared in 1993, appears in front of them.

Yoo Hyun-jae is at the hospital and notices a new invention, realizing he's jumped 20 years into the future. He returns to the rooftop of World Planning Entertainment and discovers that his room now belongs to Ji-hoon, Woo-seung, and MC Drill.

Hyun-jae joins the other three and starts living in the rooftop room. He notices that Ji-hoon calls Gwang-jae "father" and thinks he's Gwang-jae and Bo-hee's son. On the day of the civil service examination, Woo-seung has a terrible stomach ache and fails the test. On the same day, Ji-hoon is told to show off an impromptu freestyle rap at his evaluation.

Hyun-jae asks Woo-seung to take him to Lake Soyang where he disappeared. He finds an old photo with a message. On the way back, Hyun-jae stops at the store where Woo-seung works and confronts MJ for selling his song. The fans rush in. A drunken Bo-hee sleeps on the sidewalk.

Gwang-jae learns Hyun-jae's real identity, as incredulous as it is. He keeps this truth from Bo-hee and Soon-tae. Hyun-jae sings a song by J2 at a karaoke event at Dongdaemun, surprising many with his amazing voice, including Ji-hoon and Woo-seung.

Gwang-jae tells Hyun-jae his relationship to Ji-hoon and Bo-hee. However, Hyun-jae cannot recall anything. Bo-hee prepares a shrine for the death anniversary of her son just when Hyun-jae falls from the ceiling. Meanwhile, the World Planning Entertainment's building is about to be auctioned off.