From Nanya:

Kumon Sei came from a strange family where his family members disappeared one after another. Since his father used to be a famous UFO researcher, he thought UFO must have come and taken his family away and someday, he would be taken away too. Sei then found a secret door inside his wardrobe and when he opened the door, he found himself in an unknown apartment and alone with a handsome young man.

He recognized the young man resembled the child genius pianist Oribe Hiroshi whom he just saw on TV a moment earlier. The young man said he was Hiroshi but only the 18-year old Hiroshi. After going through some conclusion, Sei finally realized that he time slipped and came to the Japan 6 years later, i.e. 2004.