Widowed and in his sixties, Thomas Quinn has been asked to lunch by Lexington Smith, the leader of a covert team who call themselves The Twelve. When Quinn accepts the invitation to join the group, he finds himself united with other retired experts—from a surgeon to a locksmith to a cabbie—in a quest to capture a killer targeting homeless victims on London's transit system.

As the body count rises, Quinn and his companions try to puzzle out a pattern. But can they protect this vulnerable population by identifying a predator amid the crowds of the London Underground?