An emergency call comes  in the middle of the night, reporting an ongoing burglary at a winter-closed camping site in Bredäng outside Stockholm.
The nearest police car responds to the call. There is a light on in one of the fartherst caravans in the otherwise dark area, and when the officers open the door to the caravan, they are greeted by a horrific sight. Floors, walls, and furniture are completely covered in blood. A person has been killed with an ax and brutally dismembered.
In one of the rooms, a young man is sleeping on the floor with a severed arm as his pillow. He is arrested and taken into custody at Kronobergshäktet. There he is identified as the 17-year-old Hugo Sand, son to a famous author.
It turns out that Hugo is suffering from a rare kind of somnabulism which is triggered by nightmares. He can either be the perpetrator or a witness, but claims, nevertheless, that that he remebers nothing from that night.

Joona Linna, who is asked to take on the case, contacts his old friend Erik Marie Bark in order to use hypnosis in the quest to find out what happened inside the caravan. This becomes the starting point of a complicated hunt for a bestial killer who has just entered an extremely active phase.

The Sleepwalker is Lars Kepler’s tenth novel featuring Joona Linna.