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. Meanwhile, Grandpa-Grumble becomes obsessed with both fishing in a nearby stream that he insists is actually a brook as well as with meeting the Ancestor, a three-hundred-year-old Moomin who he is told by Mymble hibernates in the stove. 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, who is enlisted by the Hemulen into helping build a treehouse for Moominpappa, whom he is increasingly admiring.