Like Moominpappa at Sea, the work is more somber in tone than previous books in the series, and it has been suggested that this is a direct result of the death of Jansson's mother, Signe Hammarsten-Jansson, during the year that it was written.

[6] Alongside these figures, Mymble also decides to visit the Moomins in order to see her sister Little My whom they have adopted, and Snufkin also returns, realising that the valley is the place where he can gain inspiration to write a song.

Soon, their conflicting personalities begin to cause friction, with the Fillyjonk trying to tell the others what they should do: Toft finds an old microbiology textbook, and misinterpreting it as a story, creates a monster in his imagination known as the Creature, which appears to develop a life of its own.

After becoming terrified that there are insects in the house, Fillyjonk locks herself in the kitchen and, in an attempt to be more like Moominmamma and therefore liked by the others, cooks for them and tries to look after the motherless Toft,[8] who is enlisted by the Hemulen into helping build a treehouse for Moominpappa, whom he is increasingly admiring.

Toft, left alone to wait for the return of the Moomins, finally realises how the view of the family which he had developed in his imagination is too perfect to be real, and comes to accept that even Moominmamma, who he hoped will be his mother, has problems and times of anger just like everybody else.