When the body of a red-haired young man is washed up on the shore of the beautiful Isle of Mull, Detective Superintendent Lorimer's tranquil holiday away from the gritty streets of Glasgow is rudely interrupted. The body has been bound with twine in a ghoulishly unnatural position and strongly reminds Lorimer of another murder: a twenty year old Glasgow case that he failed to solve as a newly fledged detective constable and which has haunted him ever since. As local cop DI Stevie Crozier takes charge of the island murder investigation, Lorimer tries to avoid stepping on her toes. But as the similarities between the young man's death and his cold case grow more obvious, Lorimer realises that there could be a serial killer on the loose after all these years. As Keep the Midnight Out switches dramatically between the Mull murder and the Glasgow cold case two decades earlier, DC Lorimer tries desperately to catch a cold-hearted killer.