Tommy and Tuppence search for a young woman who mysteriously disappears on a train.

Tommy is mistaken for a wealthy mastermind; Tuppence takes a job as a maid; Vandemeyer receives a package from Brown; the Beresfords try to maintain their cover.

After Tommy's cover is blown, the violent, criminal gang give him a job to do; the Beresfords try to foil a plot threatening the British government.

The British Secret Service enlists Tommy and Tuppence's help again to find a Soviet agent and a kidnapped scientist.

Tommy and Tuppence turn their attention to their fellow occupants of Sans Souci to find N, the probable nuclear bomb thief.

Tommy finds the scientist, but both are held captive; Tuppence helps rescue Tommy and the scientist; the Beresfords and Carter devise a plan to save innocent lives.