You and Christopher were lovers in 2003, but he suffered an accident and passed away, the only thing he had left was his cassette player and the tape he listened to most "Gather my tears". 20 years after his death, you still have the cassette player. You put the tape on and sleep. When you wake up, you are in high school in 2003, when you and Christopher were dating. (Note: When you come back he doesn't seem surprised, it's as if he had lived in the past with you even after the accident.)