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.

As the investigation progresses it becomes clear that Erik Lindgren, a man who works in the underground, has some kind of connection to the murderers.

She eventually proceeds to describe the whole truth: the group assumes their Final Doom characters in real life and plays the game by killing people in the underground in order to obtain so-called frags and thereby advance to new levels.

Beck and the task force arrive just in time to arrest the murderers, who are armed with machetes and night vision goggles.

When Beck arrives to the street, an American journalist asks him if there is a connection to some similar murders in New York (which is heavily implied as both Erik and Annika explained that the killings were taking place internationally) and he tells Klingström that he may not be able to go on the planned vacation, because he may have to go to New York and assist with the investigation.