Kyon and Itsuki had been dating for one year. They had their anniversary date at the planetarium.

But in a moment looking away, Itsuki disappeared. Kyon couldn't contact him. All the members of SOS seemed to forget his existence. Only Kyon was remembered, using all his strength to keep him in mind. With the help of Mikuru and Nagato, he went in search of his beloved.

Another Rewind (i.e., The Disappearance of Itsuki Ko*zumi) is
a multi-ending digital novel story game about Kyon x Itsuki.

[From DLsite English]