A comet is tearing through space right towards Earth, and no one knows how to save themselves! Will it destroy everyone and everything? Moomin decides to investigate, trying to find a solution. Together with Sniff, they embark on an adventure that will surely be filled with dangers and promises.

Snufkin will meet them, and together they will reach the Observatory at the top of the Lonely Mountains to ask the wise professors and learn about the Comet. On their way, they encounter crocodiles, treasures, and the Snork siblings. And they learn that the Comet will not fall in some distant part of the Earth, but right... in the Moomin Valley!

The Moomin family, with their many and various relatives and friends who are sometimes hosted at the House in the Valley, are among the most important characters in children's literature. Their stories, written and illustrated by Tove Jansson, unfold in a magical world inhabited by imaginary creatures - yet with such recognizable human traits! Like all great works of children's literature, they are not only for children, but for readers of all ages.