In this dark age, human live under the rule of a mighty government that possesses PSYCO - a brain-washing technology that would instantly turn anyone into their willing slave. One day a young boy named Kazuya encounters Henry, a robotic leader of a resistance group that tries to overthrow the rulers. The problem is that these resistance fighters - all of them young pretty girls - have been brain-washed by PSYCO. There is only one way. Kazuya and Henry must defeat them in battle, and then Kazuya must have sex with them. That, and only that, will stop the evil influence of PSYCO. Kazuya bravely agrees. The sacrifices we are willing to make for our country's sake...

Horny Sweeper is a turn-based strategy game. The player begins the game with Kazuya and Henry alone, but afterwards more and more girls join the party after having been... erm... treated by Kazuya's wondrous psyche-restoring skills. Most of the battles are against government parties, which consist of various character classes (soldiers, mages, robots, etc.). The player's party also acquires different talents over the course of the game: a healer, a shotgun-wielding character, etc. When characters attack each other, an animated sequence is displayed.

Several stages are dedicated to defeating future female companions. Afterwards, the game switches to sex scenes, some of which have point-and-click interface and require specific actions from the player, while others utilize simple command-based menus. Some of the sex scenes are animated.