Moominmma accidentally joins a club of gangersts after being refused entry to Moominpappa's new 'rebel father's club.
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. And things only get worse for poor Moominmamma, as her wish to be admitted as a club member turns into a difficult juggling act of loyalty between conflicting organizations. Comic misunderstandings, tested allegiances, and frivolous scandals make for an exciting adventure with the whole Moominvalley gang in another classic Tove Jansson tale.