Boku wa Mari no Naka

Isao Komori is a college dropout who spends his days cooped up in his apartment playing video games. The highlight of his day is visiting a convenience store where an angel-like high-school girl called Mari visits every night.
One morning Komori wakes up in Mari's body. Another "Komori" inhabits his old body, and Mari has disappeared. Komori struggles to maintain Mari's ordinary life and starts to investigate Mari's disappearance.