S1, E4: In N or M? Major Carter commandeers Tommy for a mission he must keep completely secret, even from Tuppence. There is a leak on the Third Floor and Tommy is the only man Carter can trust. A British scientist, Gilbert Worthing, who was working on a top-secret prototype for a nuclear bomb, has gone missing from a military base in Cromer, along with his work. All Tommy knows is that the perpetrator is staying at the nearby Sans Souci guesthouse and is most likely a Soviet super spy code named N or is it M? Tommy must travel in disguise as a birdwatcher to Cromer, in order to identify which guest at the Sans Souci is holding the British government to ransom. Things get even more complicated when it turns out that Tuppence, dismayed at being left out, has beaten him to it and already ingratiated herself with the other guests. But Tommy and Tuppence must learn to work together again if they are going to identify the Soviet spy, save the missing scientist, and discover the location of the missing bomb before half of Norfolk is obliterated.