Moominpappa is dissatisfied with his life in Moomin Valley, so he organises the family to set off on a journey to find a lighthouse in the sea. This will also be the perfect backdrop for a novel about the sea. Once arriving there, they find it a desolate and lonely place, inhabited only by a very unfriendly fisherman. Moomintroll also befriends The Groke and the sea horses. Moominmamma misses home so much that she starts painting the Moomin house with lots of flowers since none can be grown on the lighthouse island. Later they find out that the fisherman is actually the lighthouse keeper who fled from the loneliness. Nature and the sea play a big part of the novel as Moominpappa tries to understand it, and there are many strange things happening on the island that seem to be inexplicable.