Moomintroll, Sniff and Little My set out to stop a comet approaching the Moominvalley. Along the way they meet Snufkin, the beautiful Snorkmaiden, Snork and Hemulen, who join them in their quest.

After a volcanic eruption causes floods, members of the moomin family find refuge inside a floating theater.

One day Moomintroll wakes to notice that grey dust is covering everything in the Moominvalley. He runs to ask the philosophical Muskrat if he knows what is happening, who advises him that things tend to look like this before an awful fate coming from the sky hits the Earth. With the help of his father, Moominpappa, Moomintroll and his close friends Sniff and Snufkin build a raft and head out on a challenging journey to the observatory in the Lonely Mountains hoping to find out more from the wise professors there. The friends have to overcome several adversities in order to make it there. When they arrive, they find the professors deep in calculations. They reveal that a comet will reach the Earth in four days, four hours, four minutes and 44 seconds.

The Moomins along with Little My and Snorkmaiden had a sea journey that after storms and desert island dangers leads the family to Riviera, the place that takes their unity to the test.

This winter is extraordinary; Moomintroll decides to stay awake to explore the winter instead of hibernating as usually. And winters are certainly totally different than he had imagined. Strangest creatures wander amidst the snowdrifts; midwinter darkness surrounds the Moominvalley, and on top of that, an eccentric guest is soon to appear. This guest requires many measures, and is called Christmas, Hemulen tells to a surprised Moomintroll.

A beautiful summer day in Moomin Valley. Suddenly, a wasp stings Moomintroll, who falls ill and has to stay in bed. To cheer him up, Moominpappa recounts his youthful adventures; his misunderstood childhood, escape from an orphanage and his historic meeting with the inventor Hodgkins.

On a frosty winter's morning, Hemulen wakes the sleeping Moomins to prepare them for the approaching Christmas. But the Moomintrolls, who normally go dormant during the winter months, have never celebrated Christmas and have no idea what the holiday means. Their confusion and delight in the white winter landscape lead to a journey of discovery, where they are forced to understand the true meaning of Christmas.