Kenta is an ordinary Japanese teenager who enjoys his high school life. One day, he receives an anonymous letter, apparently from a girl, who says she likes him, and wants to meet him in an abandoned school building. However, upon arriving there, he finds that several of his classmates and teachers (mostly female) are there as well. When Kenta realizes they were lured there, it is already too late - they are locked in the building, unable to get out. Kenta must find a way to save his friends... but what if he can't save them all?..
Though a Japanese game, Isaku is a point-and-click adventure with inventory and puzzles, resembling Western adventures more than its compatriots gameplay-wise. The game is viewed from first-person perspective; while individual locations are presented as still images, the connecting corridors are done in pseudo-3D and can be physically navigated.
Objects (and people) that can be interacted with are highlighted, offering one or more actions to perform. The gameplay mostly involves exploring the rooms and collecting the necessary information or items to unlock new areas.
The player is also able to make decisions during the gameplay concerning Kenta's relationship with the various female characters. Depending on these decisions, Kenta will be able to become romantically involved with a girl of the player's choice.
The adult scenes in the game are very rare and occur only during the ending sequence, or in case Kenta allows one of the girls to be directly captured by the evil mastermind behind their imprisonment.
