A woman is found murdered in a closed amusement park. A young couple who happen to be there at the same time become, without their knowledge, suspects for the murder. They are chased by a large number of police but only Martin Beck believes them to be innocent.

The gardener decides to avenge the death of his mentally disturbed wife by killing the men he considers responsible. One of them is a pop star that takes part in great marathon race in Stockholm. While the killer methodically prepares and patiently waits his victim, Inspector Beck and his men must determine the exact location of the shooting on 42 kilometers of path before it's too late.