This is a very good series. The male leads have a good chemistry and the series is well directed.
Jae Woo travels 15 years back in time, to try to save Ji Hoon. Ji Hoon saves 10 year old Jae Woo's life by sacrificing himself. Jae Woo is riddled with regrets. 15 years later he returns to the Dormitory that Ji Hoon had been living in, days before it is to be torn down. After being allowed to stay in Ji Hoon's old room for a little while he is transported back to the year 2007, where his 25 year old self meets Ji Hoon again. Slowly they develop feelings for each other. Unfortunately the fateful day of Ji Hoon's death arrives and having figured out that the 25 year old Jae Woo is actually the same person as the 10 year old boy he had saved weeks earlier from a kidnapper. Jin Hoon, despite Jae Woo's best efforts, decides not to change fate and again intervenes and saves 10 year old Jae Woo's life at the cost of his own!