Commissioner Maubritane answers a plea for help, and finds himself plunged into a nightmare. Pierre Sertat tells Maubritane of a mysterious intruder who keeps breaking into his house - but despite Maubritane's precautions, Sertat is shot and wounded and the visitor escapes. When more members of the Sertat family are attacked - all in locked rooms or by unseen assailants - Maubritane is driven to the brink of madness. As a last resort, he calls on M. Allou for answers. The first part of…