Neighbours Pertsa and Kilu find themselves at the scene of a crime. There has been a robbery and a yacht has mysteriously disappeared in Kotka, a town on the Baltic Sea. The two boys are ready to jump into submarines, motorboats and planes to investigate and find the missing yacht! They hope that this will not be their last adventure. Pertsa’s family faces financial difficulties and is planning to leave Kotka. If they could get the reward money for finding the yacht, it would come just at the right time!

The film is based on a series by Finnish children’s book author Väinö Riikkilä (1906–1969) about the friendship between the two boys Pertsa and Kilu. Having previously been made into a Finnish television series, Finders of the Lost Yacht is intended as the first part of a trilogy, of which the next two films are currently in development.