DI Lewis returns to Oxford after several years absence and is reluctantly assigned by his new boss, DCS Innocent, to the murder of an Oxford mathematics student who was shot while participating in a sleep study. Lewis must decipher a cryptic clue left in an old case file by his former boss, Chief Inspector Morse.

DI Lewis is less than pleased when he and DS Hathaway are assigned to protect Nicky Turnbull, a former criminal turned successful author. Turnbull had actually cheated two Oxford colleges in his computer scam and had received death threats. Turnbull is everything Lewis dislikes but he grits his teeth does the job. Turnbull is in Oxford to make a speech at the request of the Students Union but things take a serious turn when Jo Gilchrist, a student and a member of Turnbull's reception committee, is found strangled. Gilchrist wrote for a student newspaper and was about to expose a professor's exam scam. When Turnbull is shot in the courtyard of his hotel, Lewis realizes that the man's wife was his first girlfriend. He also learns that they were on the verge of getting a divorce.