Dr. Aitamaa is spending holiday with his family at their summer villa. He accidentally reads a letter that was meant to his wife. The letter is from another man. Aitamaa follows his wife to the city and finds out that she has a lover. Later the wife is found killed and Aitamaa becomes the prime suspect.
Pekka Puupää and Pätkä, who return to Helsinki from an unsuccessful variety show tour, are forced into refresher training, where they are trained as frogmen. During the diving exercises, the friends encounter the mermaid Helmi, whom they help into the bathtub at home, to Justiina's shock. The friends, who have become civilians, eventually end up on a plane bound for Suez, being cornered by UN soldiers.