Tommy is sent on a secret mission to discover the identity of Soviet spy staying at a seaside guest-house; Tuppence refuses to be left behind.

Tommy and Tuppence have a chance encounter on a train with an agitated young woman who suddenly disappears and never returns to her seat.

Tommy and Tuppence go undercover; he as a member of Brown's gang and she as a maid to a potential suspect.

The Beresfords search for Jane Finn comes to a close, as they uncover the identity of Mr. Brown.

Tommy and Tuppence turn their attention to their fellow occupants of Sans Souci to find N.

Tommy has finally found the missing scientist, but they're both being held against their will by M, and still none the wiser as to who N is.