Geppy-X is a 2D side-scrolling shoot-em-up developed as an affectionate parody of the 1970s super robot era of mecha anime pioneered by Go Nagai and Ken Ishikawa. In it, you play as a super robot fighter squadron in an alternate universe 1970s, fighting against alien invaders from the Space Devil Empire.

The titular robot has three transformations all modeled after famous 1970s super robots, as well as a super form known as Geppy-XX. Stages are presented like actual anime episodes, and the game makes use of the PlayStation's full motionn video capabilities to include actual animations made specifically for the game, with an opening, part A, eye-catch, part B, ending theme, and next episode preview. They even have fake commercials that play in between the eye-catches.

Is the year 1970s on earth, and suddenly the devil space empire Beelzebub beast space corps led by Bin Day began their invasion.

The invasion was predicted by a space robot laboratory called Wu, and he was secretly building Super Robot called "Yuck PX" that is piloted by a human called Hazime Kei (17 years old) that is the leader of the red squadron.