One day, Osomatsu's five brothers went missing. The mischievous Iyami, of course, must be behind this! With great haste, Osomatsu grabs his slingshot, and sets out to rescue his brothers and give Iyami a taste of his own medicine.
Osomatsu-kun: Hachamecha Gekijō (Nonsense Theater) is a side-scrolling action platformer based on the manga and anime created by Fujio Akatsuka, which plays similarly to Alex Kidd. The player controls the titular Osomatsu through outlandishly Japanese environments, fighting bizarre enemies, collecting ribbons to buy various items from hidden shops, and searching for his missing brothers. Each stage is made up of multiple areas; falling into holes in the ground leads to underground passages, and jumping onto flying shells leads up into the clouds. A boss waits at the end of each stage, usually just Iyami in some sort of strange disguise.
Description from MobyGames