Seokjin lived in America for a large period of his life. He came back to Korea at 19, where his father wanted him to go to school. The school was the same that his father went to. His father controls him, and the principal uses that to his advantage. He forces Seokjin to tell him about the boys, in order to be a good student. Seokjin doesn't want to deal with that, and tells the principal about their hideout. He thought no one would be there, but Yoongi and Jungkook were.
Seokjin became seperate from the group after that. Seokjin decided to tell Namjoon about what he had done, also saying that he had told the teahers that Yoongi and Taehyung were getting into fights. Taehyung, however, overheard Seokjin telling Namjoon, and wasn't happy. Seokjin left for America again soon after.
Seokjin returns 2 years later. On April 11, he goes to the sea where they met, missing his friends. He sees Namjoon and Jungkook on his way back, but doesn't speak to them. On May 22, when he goes to find them, he hears news from Namjoon. Jungkook and Yoongi are dead, Hoseok had an accident, and he doesn't know where Taehyung and Jimin are. (Taehyung is being arrested, Jimin is in the hospital.) After this, he travels back in time to April 11 for the first time.
Seokjin travels back to April 11 an unknown amount of times. At first he doesn't realize what is happening, but he figures it out and trys to save each member. He soon becomes unsure if he can save them all.