Martin Beck is on his way to a vacation with his colleague and girlfriend Lena Klingström when several gruesome murders in the Stockholm Metro force him to return. Without any kind of motivation, he is forced to lead the investigation of the strange murders. There are four things connecting the murders: they are all in the underground, they are all decapitations, and a sign is left by the murderer/murderers. This sign is "X-(", which means "death" in Leet. The last connecting thing is that before every murder, all lights have been turned off (note that this was faked and in reality impossible).