Kaede is a cheerful and energetic eighth grader, but when it comes to boys, she is hopelessly shy. One day on her way home from school, she wanders into a mysterious shop and buys a colorful cocoa mug. At home, she casually peeks into the bottom of the mug and finds an engraved note that says, "If you read this message aloud while pouring hot cocoa into the mug, a love fairy ('muglox') will appear and grant your every wish." Though skeptical, Kaede follows the instructions and announces her wish to date Yuuki, the class heartthrob. Suddenly, the adorable blue Mirumo appears! But it quickly becomes clear that this cute little muglox would rather eat chocolate and cause mischief than help Kaede.
Mirumo, it turns out, is prince of the muglox world. Horrified at the prospect of having to marry Rirumu, his princess bride-to-be, Mirumo has escaped from the muglox world. Hot on his trail, however, are Rirumu, Yashichi the bounty hunter, and a cast of hundreds of muglox ranging from good to bad to nutty. This gang of adorable troublemakers ensures that school life for Kaede and her friends will never be the same.