After seven years of ducking the question, Lewis and Hobson are embarking on a relationship, though the detective has a new puzzle to solve. An elderly don, Richard Seager, is struck by a car on the night of his release from prison; before dying, however, he mysteriously scratches the number '500' into the paintwork. Lewis must contend with Seager's wife, convinced that the sister of her husband's victim is the guilty party.