Hampstead Heath, London. A dead body is discovered in the undergrowth. The victim, a young, white girl was stabbed several times in a frenzied attack. The only clue is a handkerchief monogrammed with the initial 'S'. Given the lack of blood at the scene Superintendent Robert Hazard speculates to his colleague Inspector Phil Learoyd that she must have been killed elsewhere and her body dumped on the heath.

The victim turns out to be a music student named Sapphire. The police search for her murderer begins with her boyfriend David Harris, a gifted architecture student and the recent beneficiary of a scholarship to a European University. But Harris claims he was in Cambridge on the day in question and did not return until 11pm. Nevertheless, Superintendent Hazard feels that he is concealing something.

An autopsy reveals that Sapphire was three months pregnant. When her brother, Doctor Robbins, arrives from Birmingham the mystery deepens. Doctor Robbins is a black man. It seems that their father was a white doctor and their mother a black dancer. Though the Superintendent is courteous, Learoyd cannot disguise his revulsion. It appears that the motive for her murder was racism.

As the police investigate Sapphire's history, it emerges that she lived a double life: passing for white in one guise while frequenting some of the shadier nightclubs in black neighbourhoods in another. A torn picture found in her boarding room sparks the hunt for a missing dance partner. Flashy clothes suggest a dubious lifestyle.

Meanwhile Hazard and Learoyd call on the Harris family. David lives with father, mother, sister Mildred, and Mildred's young twin daughters. Sapphire had visited the family on Saturday morning and told them that she was pregnant. The father agreed to a marriage though it is clear that no-one in the family was very happy with the situation - the family knew that Sapphire was coloured. According to the beat policeman Ted Harris, the father has a reputation as a bigot and racist, as does the daughter. Hazard suspects that David killed the girl to protect his scholarship, but Learoyd is inclined to believe his innocence.

Dr Robbins wants to see the spot where his sister was murdered and to visit her boarding room. But the landlady of the lodgings will not let him in the house because of his colour for fear that she would lose potential lodgers.

Hazard and Learoyd visit an upmarket club for young, well to do black people. Sapphire used to frequent the club with an ex-boyfriend, Paul Slade. However, Sapphire preferred the racier environment of the Tulip club, where she began dating a big man called Johnny. Many of the witnesses questioned by Hazard and Learoyd resent Sapphire either because she passed for white or because she is discovered to be coloured.

At the Harris's home, Mrs Harris questions her son closely, but he refuses to say where he was on Saturday night. While mother has her suspicions, father and sister defend David. The whole family are fearful of the social consequences should their association with Sapphire be made public.

After a pursuit through the streets, where Johnny is refused help or shelter and battered by racist Teddy boys, the police eventually corner Johnny. Johnny's room is searched by the police, who find a bloodied sheet and a flick knife. It also appears that he is the missing man from Sapphire's photograph. Johnny claims he had a fight with another club regular, Horace Big Cigar.

Meanwhile David's alibi is beginning to crack. He arrived back in London earlier than he claimed, and he was observed acting suspiciously on the Heath, after the murder. The Harris's house is searched. Superintendent Hazard invites Doctor Robbins to the house, much to the disgust of the family. When Robbins arrives Mildred explodes with racist venom, revealing her pathological hatred of black people. She incriminates herself as the murderer.

Mildred's motive was the desire to save the family from disgrace, spurred by Sapphire's taunting about the race of the baby she was carrying. Superintendent Hazard leads Mildred away to a waiting police car. Doctor Robbins returns to Birmingham.