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. The key-code used to access the sleep lab was assigned to a fellow maths student, Daniel Griffin, but Daniel's math tutor has provided him with an alibi. Daniel is a maladjusted young man who will soon inherit his father's automotive empire. He is disruptive and has no respect for his uncle who now heads the company. The future of the company however rests on an impending deal with Japanese investors who insist that family unity is all important at this time. When two other murder occurs, Lewis must decipher a cryptic clue left in an old case file by his former boss, Chief Inspector Morse.