A little boy named Suk receives an unexpected visit from a fairy tale superhero named Koo. Koo explains that the fairy tale worlds are in grave danger: the heroes were corrupted and turned into villains. The only way Suk and Koo can stop them is by merging with each other and becoming an even more powerful fighter who can put an end to the trouble in fairy land.

My Friend Koo is a colorful action platformer. The protagonist travels to various worlds loosely based on popular stories for children such as Peter Pan, The Wizard of Oz, and others. Sometimes it is necessary to talk to friendly characters and acquire items. Most of the game, however, is dedicated to traditional platform jumping and fighting using punches and kicks. The hero also learns several magic spells during the course of the game, and gains access to special attacks such as summoning a unicorn that flies around and kills enemies.