The story begins a few weeks after the events of The Moomins and the Great Flood, as the Moomin family are settling into their new life in Moominvalley. Sniff, who is now living with the Moomins, discovers a mysterious path in a nearby forest. As he and Moomintroll explore it, they meet the mischievous Silk Monkey and arrive at a beach, where Moomintroll goes pearl-fishing. Meanwhile, Sniff and the Silk Monkey find a cave, and the three decide to hide Moomintroll's pearls there. The next day, as they go back to the cave, they find the pearls arranged in the shape of a star with a tail. Back at Moominhouse, the Muskrat, a philosopher whose home was ruined by Moominpappa's bridge-building and who is now staying with them, explains that the pearls depict a comet. He directs Moomintroll to the Observatory on the Lonely Mountains, where the Professors would be able to tell him whether the comet will hit the Earth.