If you are looking for the episode... see Last Bus to Woodstock
Two girls are waiting for a bus to Woodstock, and decide to hitch a lift. Later that night, one of them, Sylvia Kaye, is found murdered and apparently sexually assaulted in the car park of the Black Prince pub in Woodstock.
Suspicion falls on various characters. The body is reported found by John Sanders, a young man who, it later transpires, is addicted to porn and sometimes paid Sylvia for sex. He admits to waiting for her on the night of her murder but found her dead. It turns out he interfered with the body but did not murder her.
Inspector Morse discovers the lift was offered in a red car and guesses various bits of information about the owner. This leads him to calculate the chances of finding a red car in North Oxford which meets all the criteria. There is only one, and it belongs to Bernard Crowther of Southdown Road. Crowther admits that although married he was having an affair with another woman. He admits giving a lift to two girls and dropping them in Woodstock while on the way to meet his mistress.
Crowther and his wife kill themselves, each thinking the other one was the murderer. In the end it turns out it was neither of them but the other girl at the bus stop, Sue Widdowson, who was Crowther’s mistress. Crowther had dropped her off and had sex with Sylvia. Insanely jealous, the other girl had crept up behind Sylvia in the car park and banged her on the back of the head.
A sub plot is about Jennifer Coleby who worked with Sylvia in an insurance office. She was having an affair with her boss, Palmer, and shared a flat with Sue. Each helped the other with messages for their affairs, and the police are sent on a merry dance chasing them.