Gunvald Larsson sits carefully observing the dingy Stockholm apartment of a man under police surveillance. He looks at his watch: nine minutes past eleven in the evening. He yawns, slapping his arms to keep warm. At the same moment the house explodes, killing at least three people. Chief Inspector Martin Beck and his men don't suspect arson or murder until they discover a peculiar circumstance and a link is established between the explosion and a suicide committed that same day, in which the dead man left a note consisting of just two words: Martin Beck.