Two Finnish PI:s are hunted by a gang specialized to counterfeit banknotes from Malaga to Finland and Mallorca and back. The counterfeit money printing plates are desperately searched by the gang and our heroes try to hide from them by travelling to Mallorca, where some of the local hoods try to kill them and retake the missing plates.
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.
An architect, who has recently married a new, younger wife, tries to re-connect with his ex-wife, who is being courted by another man.