In his first book Bunso Meets a Mumu, Bunso's antics get him into trouble and his family warns him that the Mumu will get him if he doesn't behave. Bunso wonders what a Mumu is, and takes it upon himself to capture the Mumu and find out. What he discovers is that the Mumu is actually a spirit that never really got to enjoy life and have fun. Bunso, wishing to help the Mumu, teaches him on how to do playtime. Having learned how to play, the Mumu...