Yonaka Kurai, an extremely ordinary high schooler. She takes the usual train home.
Though it's all ordinary, today is somewhat special. Today, her beloved brother comes home.
But after waking from a nap, the train has stopped at an unfamiliar station...

WARNING:
Deep-Sea Prisoner rates this game ages 15 and up.
(This rating is Japan-centric and would likely be more strict by other countries' standards, 18 and up may be more appropriate.)
It is very violent and contains instances of implied sexual assault, with the bad endings being some of the most severe parts.
I would advise against playing if you think you may be upset by such content.
Deep-Sea Prisoner also insists that players should be able to keep reality and fiction separate.