Sung Hae Sung reappears 12 years after a fatal accident, unchanged and seeking answers about his resurrection. Aided by friends and Jong Won, he strives to right wrongs and unravel the enigma surrounding his death.

Sung Hae Sung (Yeo Jin Goo, Circle: Two Worlds Connected) is a cheery high school student. Being the eldest sibling, he wants to provide for his family instead of going to college. On the day of his 19th birthday, he gets into a car accident and passes out. He wakes up on the top of his school building. He thinks he survived and everything is normal. Except everyone thinks he died 12 years ago, and he hasn't aged at all. Jung Jung Won (Lee Yeon Hee, The Package) is Hae Sung's best friend. She was the one who took him to the place where he had his accident. Distraught, she has been unable to move on from the incident since. Having failed to pursue her dreams under the stress, she works as a kitchen assistant at a nice restaurant. Cha Min Joon (Ahn Jae Hyun, Cinderella and Four Knights) is the owner of the restaurant where Jung Won works. He has feelings for her. However, she has never shown any interest in him. In her mind, she can't be with anyone while carrying Hae Sung in her memories. One night, Jung Won finds a young man passed out in front of her house. He is Hae Sung, her first love. She is now 31; he is 19. Can they pick up their love where they left off, or will their reunited worlds always stay separate? Also known as "Into the World Again" and "Into the New World," Reunited Worlds is directed by Baek Soo Chan and written by Lee Hee Myung, the creative team behind Beautiful Gong Shim and Sensory Couple.

Hae Sung (Yeo Jin Goo) disappears after a mysterious accident. 12 years later, he wakes up on the roof of his school building. He is now 31 years old, but he still has the appearance and personality of himself at age 19 when he disappeared. He reunites with Jung Won, his childhood friend and first love. The two of them end up being 12 years apart because Hae Sung was gone to another world. Hae Sung's siblings were all separated after his death, so he brings them together. He later finds out that he was unjustly accused of killing someone after he died. To solve the mystery of his death and clear himself of a false charge, he begins his own investigation. Accompany him on this fantasy romance journey to find out where he will be at the end of the road.