A composer (Laurence Payne) is stuck in a middle-class marriage and finds that his affair with his wife's half-sister (Jane Griffiths) has resulted in a pregnancy. When his wife refuses to give him a divorce he hatches a murder scheme that is too clever by half.

Wartime thriller with film noir elements based on a true story as written in A.P. Scotland's autobiography "The London Cage". The plot has greatly exaggerated the actual events of A.P. Scotland's experiences, including the addition of a fictional love interest.