Kyousuke comes to Riki’s classroom one day, telling him something terrible has happened. He takes him to another room and they discover a truly awful sight…
Nishizono Mio’s corpse.
No, she isn’t actually dead. But there’s something very strange about this, namely the fact that Mio was "murdered" in a locked room. Riki is promptly tasked with solving the mystery of how this could have been possible.
[From Rincchi's blog]
This game was released with the first Blu-ray Disc volume for the Little Busters! Refrain anime.