On a cold and rainy Stockholm night, nine bus riders are gunned down by a mysterious assassin. The press portray it as a freak attack and dubs the killer a madman, but Superintendent Martin Beck thinks otherwise. One of his most ambitious young detectives was among those killed, and he suspects that it was more than a coincidence.
Working on a hunch, Beck seeks out the girlfriend of the murdered detective, and with her help Beck reconstructs the steps that led to his murder. The police comb the country for the killer, only to find that this attack may be connected to a case that has been unsolved for years.