In this haunting portrait of a diseased society, the police are unable to catch a child-murderer so other criminals join in the manhunt. Anxious to capture the elusive child killer Hans Beckert (Peter Lorre), police begin rounding up every criminal in town. The underworld leaders decide to take the heat off their activities by catching the murderer themselves. Once the locals identify Beckert, they mark him by chalking the letter M on his back.