In this film Beck and his team are trying to find a serial killer, whose victims are spread all over Sweden. An obvious connection is that all the victims are women, and they were all in the same class, and Beck thinks that a scandal back in the past has something to do with the murders. But no one wants to talk about the past and it becomes a struggle to find the perpetrator before the final murder - only one woman is still alive.

The Stockholm police bomb squad blows a briefcase to pieces after what looks like false alarm, but a baby was hidden inside, which is thus executed by shocked cops. As if the PR disaster weren't bad enough, the press gets wind of enough to cry cover-up. Soon after, young mother is targeted. Beck tries to link her to the baby by its biological father Ove Lundin, thus the prime suspect, but still lacks a motive for the sadistic MO, and the fiend 'Monster' isn't done.