All in the Mind

Lancaster, Northwest England, the naked body of a woman is found lying across a railway line in the middle of the night. The site is near a spot where couples go for casual sex at night, and it is in the vicinity of a large mental hospital. Police are totally unable to identify the victim, and there are no clues as to exactly what happened.
Eventually a report of a missing woman leads to an identification, but there are still remarkably few details or clues available - it is as though she was not a real person. Further investigation reveals a different identity, which leads to a number of possible suspects but the police are unable to incriminate a single one of them.
The investigation is helped by a young woman who had suffered domestic abuse, a young man who is somewhat socially disconnected, a pest control
officer in a large mental hospital, and a long-suffering police sergeant, amongst others. Each provides a small amount of evidence over time, and their lives gradually intertwine in friendship.
But one person has known all along. She knew who had committed the murder, where and when, but she is not able to tell anyone about it. In frustration she takes her own dramatic action.