The body of a pregnant young woman is found stabbed on Hampstead Heath. Although she appears to be white, when her brother (Earl Cameron (actor)) arrives at the police station to give evidence, the investigating police officers see that he is black. He confirms that he and Sapphire were both the children of a white father and a black mother, but Sapphire has recently been passing for white. Sapphire's white boyfriend, a student, immediately becomes the chief suspect, but, as the investigation proceeds, other aspects of Sapphire's life in London bring to light other possible suspects.