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.

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