Lucille (Ursula Howells), the heartless wife of banker Robert Ainsworth (Griffith Jones) is engaged in two extra-marital affairs with penniless artist Clive Franklyn (John Van Eyssen) and married bookmaker John Langford (Philip Gilbert). Her husband becomes suspicious and begins to follow his wife, ultimately to a rendezvous on a dusky Hampstead Heath. But just as he is about to confront his wife he slips and is apparently knocked unconscious. Later that evening the body of Lucille is found strangled on the Heath and Detective Inspector Marshall (Ewen Solon) is put in charge of the investigation. Robert resists telling the police he had been following his wife and relies on fellow banker Gilbert Morgan (Carl Bernard) to provide him with a bogus alibi while he sets about proving his innocence.