In 1993 a sixteen-year-old girl is found dying by the side of the road. Her killer disappears into the night. Her name is Claire Tiltman, and as police investigate they learn she's a girl with no enemies, and the crime is motiveless. There's no sexual attack, and no robbery. But there is extreme violence. Two years later, another attack is committed on a young woman. This time, locals identify the town milkman, Colin Ash Smith. Police investigate, and find a diary with details of crimes he's committed and crimes he plans to do. In his car they find half a school-tie, part of a weapon he used on an earlier crime seven years earlier. He's jailed for two attacks, but not Claire's murder. It takes over twenty years before he's finally brought to trial, following advances in forensics, and found guilty of killing Claire. Jurors hear of how he knew Claire, and even went to her funeral.

• Katherine Kelly