An earlier story The Moomins and the Great Flood exists, but Comet in Moominland is often seen as first novel in the series as it introduces many of the main characters, like Snufkin and the Snork Maiden, and is set in Moominvalley.
The English language book is a translation of the first version of Jansson's book, which she was later to revise, which was published in 1968. It contains a number of minor differences; for instance, the Silk Monkey character is changed to a kitten.
They go pearl fishing, and then discover a cave. However, they get scared when they see a comet in the sky above Moominvalley, which the Muskrat tells them may well destroy the valley.
So they sail to the observatory in the Lonely Mountains to ask the Professor if the comet will destroy the Earth.
They are attacked by crocodiles who eat their trousers. They then meet Snufkin, who joins them, and Sniff has a run in with a giant lizard after he tries to steal its shiny pebbles.
The boat gets stuck in a cave and they almost fall into a dark hole when they are rescued by a Hemulen. They find the observatory, and the scientists tell them the exact time that the comet will arrive.
Moomintroll saves the Snork Maiden from a deadly bush, then she and her brother Snork join them on their way back to Moominvalley.
They buy gifts in a store, then attend a party in the forest. The comet heat dries up the sea, and the group has to cross on stilts.
On the way they meet the Hemulen again, who follows them home. Moominmamma has baked a welcome home cake.
The family flees to a cave, along with the Muskrat, Silk Monkey, and the Hemulen. However, the comet misses Moominvalley. The friends rejoice.
The book was made into an animated Japanese movie, Tanoshii Moomin Ikka: Muumindani no Suisei .