Someone is killing young girls in the once-peaceful parks of Stockholm - killing them after having his own way with them. The people of Stockholm are tense and fearful. Police Superintendent, Martin Beck has two witnesses: a cold-blooded mugger who won't say much and a three-year-old boy who can't say much. The dedicated work of the police seems to be leading nowhere, and with each passing day, the likelihood of another murder grows. But then Beck remembers someone - or something - he overheard.