Agatha Christie is best known for her many spellbinding murder mysteries, but she occasionally departed from that formula. The 1941 espionage thriller N or M? is just such a departure, despite the fact that it features two detectives who have previously made a name for themselves by solving mysteries.

Tommy and Tuppence Beresford are back once again, this time middle-aged with two grown children, and looking for something to do to help their country during the early stages of World War II.

Plot summary (with possible spoilers): The book opens with Tommy and Tuppence lamenting the fact that they’re viewed as “too old” to be of any use during the war. Tommy tried to enlist in the armed forces, but was rejected in favor of younger men, and Tuppence had the same experience when she tried to sign on as a nurse. So now they’re relegated to sitting around their home, while seemingly everyone else, including twins Deborah and Derek Beresford, are allowed to take a more active role in the war effort.

Then one day Tommy gets a visit from Mr. Grant, their former contact when they were doing detective work as a young couple. Mr. Grant has a position open for Tommy. Ostensibly, it’s a desk job doing mundane paperwork in Scotland, but as soon as Tuppence leaves the room, Grant reveals the true nature of the assignment.

Tommy is to go to a boardinghouse called Sans Souci in Leahampton, where it’s believed that two high-level German spies have set up shop in order to help plan a major infiltration. British intelligence agencies don’t know anything about the spies except that one is a man and one is a woman, and that they’re referred to only as N and M.

Since there’s reason to believe that even the British agencies have been compromised by Nazi sympathizers, Mr. Grant wants to bring in a trusted outsider to check out Sans Souci and see what’s going on. Tommy fits the bill. Unfortunately, due to the extremely dangerous nature of the assignment, Tuppence will have to stay at home.

Tommy readily accepts, though he knows that Tuppence will regret being left out. What neither he nor Mr. Grant realize, however, is that Tuppence was eavesdropping on their conversation when they thought she was out of the room. She then makes haste to Leahampton and settles herself into Sans Souci as a widow named Mrs. Blenkensop, and when Tommy, posing as a Mr. Meadowes, arrives, he has no choice but to allow Tuppence to help him on the sly.

The rest of the novel then shows Tommy and Tuppence getting to know the other boarders at Sans Souci as they try to determine which two are actually German agents. They make a few missteps along the way, are mistaken in several of their assumptions, and both face life-threatening situations before the identities of N and M are finally unraveled.

My Reaction: N or M? was a surprisingly engaging read from start to finish. I usually only find Christie’s murder mysteries to be page-turners, but this one was interesting, fun, and suspenseful all at the same time.

I think a lot of the credit for this has to go to the two main characters, Tommy and Tuppence. They’re such a great pair and play off of each other so well that it’s hard not to like them. Tuppence was especially good in this one, as she manages to outshine Tommy yet again. I loved how she was able to think on her feet while held at gunpoint at the end — even if the whole story wasn’t quite clear to her yet. She bought the time she needed, and escaped with her life.

For my part, I had no idea which male character was the German agent, but I guessed the female conspirator pretty early on. There were several clues leading to this assumption, including the way two of the other female characters were portrayed in such a suspicious light. I knew Christie was just trying to throw readers off the track there!

A second clue was the fact that a child was so central to the story. Christie wouldn’t have put that character in the novel if she wasn’t important. And finally, once Mrs. Sprot took dead aim and picked off the would-be kidnapper even though she said she’d never handled a gun before, I knew it was her.

Overall, I thought N or M? was one of the better Christie books I’ve read. It may not be a murder mystery, but it’s still an engrossing read. Definitely recommended!