Moominmamma joins a club of robbers, but will her unshakeable moral code betray her?

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 Club Life in Moominvalley an exciting adventure with the whole Moominvalley gang in another classic Tove Jansson tale.