On a rainy evening in Moomintroll's world, the story begins with Moomin's father completing the construction of the cozy Moominhouse. That night, an unexpected visitor, Muskrat, arrives at the doorstep seeking shelter because his riverbank home has been washed away due to the recent construction of a bridge by Moominpappa earlier that day. The following morning, the weather shifts dramatically; a darkening rain turns everything surrounding them black, and Muskrat warns the group that the soot is from outer space. He fears that a menacing threat is headed toward their world, although he feels powerless to stop it.
Eager to find a solution, Moominpappa suggests they visit an astronomical observatory located in the remote Lonely Mountains, where a powerful telescope can peer vast distances into space. As they set out on their journey, they encounter Snufkin, a wandering, free-spirited character, for the first time. Snufkin has heard of a mysterious comet that might collide with Earth anytime soon, and upon learning about it, he eagerly joins the group. As they climb the steep, rugged mountains toward the observatory, Moomin discovers a shimmering golden anklet. Snufkin immediately recognizes it as belonging to Snorkmaiden, adding a personal touch to their adventure.
After reaching the observatory, the group learns from the astronomers that the impending comet will strike Earth in just two days. As they rush back home, an exciting incident occurs—Moomin rescues Snorkmaiden from a giant carnivorous plant, and during this act of heroism, she falls in love with him. With their bond strengthened, both Snorkmaiden and Snork join the rest of the friends as they race against time to prevent the catastrophe. Along the way, they encounter Hemulen, who joins their effort, and when a fierce hurricane hits the area, everyone takes refuge in Moominhouse using Snufkin's portable tent.
During the chaotic escape, Sniff becomes separated from the group after spotting a cute kitten, which he follows while clutching a piece of cake. As the comet's impact approaches, only moments remain, and a desperate search ensues. Moomin and Snufkin eventually find Sniff, who is frozen with fear, and together, they carry him into a safe cave for shelter. Miraculously, just as the comet looks set to collide with Earth, it suddenly changes course, veering away and preventing catastrophe.
The following morning, the natural world begins to heal as the sea returns to its usual state. The characters celebrate their survival on the beach, where Moomin presents Snorkmaiden with a beautiful pearl he discovered during the adventure. The story emphasizes hope, friendship, and courage in the face of the unknown, showcasing how teamwork and quick thinking can save the day even when faced with the greatest fears from beyond the stars.
