Elderly genealogist Adrian Weiss is murdered with an Indian ceremonial dagger in an Oxford museum. Other visitors that day were a party of school-girls from Blythe Mount school, where Morse goes, to find two teachers and seven girls staying there for the summer vacation. After he has left he finds a note reading 'Save Me' stuffed into his pocket. He learns that exactly a century earlier the school, then a private house, was the scene of the unsolved murders of the Blaise-Hamilton family, former owners of the dagger. Soon afterwards sensitive pupil Bunty disappears after hearing ghostly music played in the night. Headmistress Miss Symes tells Morse that the spectre of Charlotte Blaise-Hamilton is said to haunt the school and both Morse and teacher Miss Danby believe they have seen her. Morse contacts Stephen Fitzowen, who wrote a book about the murders and who reports that Weiss was keen to see him before he died and Fitzowen and Morse mount a ghost watch, during which there is another murder. By studying the earlier case Morse works out who killed the family and discovers that an illegitimate descendant is now at work in present day Oxford.