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 classic 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, who is convinced that the sister of her husband's victim is the guilty party.