After being told that only "rebel fathers" can be admitted to Moominpappa's new club, the Knights of the Catapult, Moominmamma defiantly decides to join a club of her own. Unfortunately for her, she accidentally joins a club of gangsters who revel in dubious and illegal activities. Comic misunderstandings, tested allegiances, and frivolous scandals make for an exciting adventure with the whole Moominvalley gang in another classic Tove Jansson tale.