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.

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

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.