Pentonville Prison. Wally Hibbert is serving a long sentence for arson. But after befriending and tricking one of the officers, Hibbert makes an audacious escape. Inspector Marmion, the detective who arrested Hibbert, is warned to watch his back, but it seems that Hibbert has another target in his murderous sights. However, the investigation is mired in confusion, the identities of killer and victim become increasingly ambiguous. An inmate at an internment camp who might be a spy sending intelligence to the Germans complicates matters further, and the multiplying manhunts, as well as Marmion's concern for his injured and withdrawn son Paul, leave the detective desperate and perhaps with too many threads to untangle.