Albert, the Beresford’s factotum and office boy who spent too much time watching gangster films in the cinema, also makes a welcome reappearance, although for fans he is sadly underutilized. There are delightful touches in this novel, including the fondly disrespectful way in which the Beresford twins, Derek and Deborah, refer to Tommy as “Carrot Top” for his still-ginger hair, and how they patronizingly look on their parents as having been in some sort of previous adventures, but clearly being a bit past it at the time of writing. Surely, the obvious choice of the young German refugee chemist, Carl von Denim, must be the right one?

For that matter, the neighboring house of Smuggler’s Rest, under the command of retired Naval man Commander Haydock did in fact have a German spy in it, but he had already been rooted out, so where could the problem lie? It is up to Tommy, in the guise of inoffensive ex-soldier Meadowes, to determine who the threat is, and how to stop them. Yet the boarding house of Sans Souci appears to be full of almost nothing more than the usual cast of middle class English eccentrics, albeit with an Irish landlady with a shady past of her own. Tommy’s brief is to determine the identities of two German spies, known only by their code letter of “N” or “M,” one being male and the other female. When at last Tommy is given a job investigating a guesthouse in Leahampton which may be host to some enemy activity, Tuppence, who isn’t included in the assignment, is nevertheless first on the scene, as the widowed Mrs Blenkensop. And what better way to carry it off than by allowing the full toll of age and years to have fallen on the couple? Rather than making the Beresfords seemingly immortal, we find the youthful pair now in their mid-40s in the second year of the war, with grown children who are both “doing their bit” for the war effort, but the elder Beresfords themselves unable to secure any war work, despite their previous connections with Intelligence. The return of Tommy and Tuppence Beresford during the chaos and uncertainty of the early days of the Second World War made perfect sense for Christie: bringing back the carefree adventurers who had tackled a post-Great War problem in The Secret Adversary, then defeated a mysterious foe in Partners in Crime, was a way to hearken back to days of youthful innocence.

Agatha Christie, N or M? (Dell Publishing, 1971)

The secret adversary book summary