Personal tragedy strikes as Morse's niece, Marilyn Garrett, commits suicide. Morse was very fond of her and had hopes that she would accomplish many of the things he had not done in his life. While Morse is away on personal leave, Lewis is studying for his inspector's exams and is temporarily assigned to Chief Inspector Holroyd, a by-the-book type of officer. They investigate the death of a young man who had been walking in a tunnel and was struck and killed by a train. When Morse and Lewis are reteamed, they see similarities in the cases and look into the possibility of drug use at all-night dance parties in the area.