N or M?

Now the Major TV Series Partners in Crime
Set during World War II, Agatha Christie’s riveting novel in her Tommy and Tuppence series follows the unlikely espionage agents as they pursue a pair of Nazi spies who have murdered Britain’s top agent.
World War II is raging, and while the RAF struggles to keep the Luftwaffe at bay, Britain faces a sinister threat from “the enemy within”—Nazis posing as ordinary citizens.
With pressure mounting, the intelligence service appoints two improbable spies, Tommy and Tuppence Beresfordac. Their mission: to seek out a man and a woman from among the colorful guests at Sans Souci, a seaside hotel. But this assignment is far from an easy stroll along the promenade—N and M have just murdered Britain’s finest agent and no one can be trusted.
“Any mystery writer who wants to learn how to plot should spend a few days reading Agatha Christie.”—Donna Leon, internationally bestselling author of the Commissario Guido Brunetti novels

Praise for Agatha Christie: “Agatha Christie proves that if you really are good enough, you can break all the ‘rules’ and still come up with a winner.” — Anne Perry, New York Times bestselling author of the Thomas Pitt and William Monk series of Victorian mysteries
“As ingenious as ever.” — The Guardian