London, 1968. A young woman is found naked and strangled in an alley in well-to-do St John's Wood. The neighbours would love to pin it on the enigmatic black stranger who has just moved in. Detective Sergeant Cathal Breen is convinced there's more to the case than anyone wants to admit; no-one's listening. In walks WPC Helen Tozer – awkward chatterbox, farmgirl, and the first woman to enter the murder unit – and gives Breen a breakthrough.