While in the Lake District, journalist Emmeline Kirby and jewel thief/insurance investigator Gregory Longdon overhear a man attempting to hire international assassin Hugh Carstairs, a MI5 agent who went rogue. They race back to London to warn Philip Acheson of the Foreign Office and Superintendent Oliver Burnell. But it's a devil of problem to prevent a vicious killing, if the target is a mystery.

More trouble brews as Emmeline pursues a story about shipping magnate Noel Rallis, who is on trial for murder. Rallis is desperate to keep the negative publicity from exposing his illicit schemes, especially something sinister called Poseidon. Lord Desmond Starrett, whose dark past made him easy prey for blackmail, is getting cold feet about their dubious partnership. Hovering in the shadows of this ugly secret world is a Russian mole buried inside MI5. Scorned prima ballerina Anastasia Tarasova makes the fatal mistake of threatening to reveal all she knows. The hunt for the answers takes Emmeline and Gregory up to Scotland, where they learn that the truth has lethal consequences.