From MangaHelpers:

There's a new wheel-chair bound student at school who has returned after an illness, and she immediately kisses Tsuzuki Ui, who she calls "Rei-kun." He soon deduces that she is a daughter of a wealthy family, and a person he once beat at an internet murder mystery game. She is apparently fixated on him, since he was the only one who ever beat her, and she has now come to see him in person. Her plan is to have a real-life murder mystery game, and her current focus is on the supposed suicide of Tsuzuki's father. For some reason, she suspects his little sister Haruka... [tethysdust]