第 1 季，第 1 集: 1952. Married couple Tommy and Tuppence have a chance encounter on a Paris train with an agitated young woman, named Jane Finn, who suddenly disappears and never returns to her seat. Back in England, Tommy is focussed on their fledgling honey business, Tuppence is much more concerned about the missing girl and her investigations bring them into contact with Tommy’s uncle, and Third Floor bigwig, Major Anthony Carter. It turns out that Jane Finn was carrying a secret recording that would potentially reveal the identity of a legendary Soviet assassin, known only as Mr Brown, who they believe will strike soon in Britain. Carter believes Brown’s cronies must have kidnapped Jane - they can only hope she managed to hide the recording before they got to her. Despite Carter’s attempts to keep them out 54 of it, Tommy and Tuppence soon find themselves compelled to infiltrate Brown’s gang in the hopes of finding Jane Finn, the recording and foiling Brown’s next hit.