The book opens a couple of years after WWI. Sir Richard Hannay and Mary Lamington are now married with a young son. They reside in Fosse Manor, a nice touch given they first met when she was staying with her aunts at the Manor while engaged in a bit of undercover work herself, as related in Mr Standfast.
One evening the Hannays' friend Dr Greenslade visits and their conversation turns to how to write what he calls a "shocker". Dr Greenslade's theory is the author should take three apparently unconnected things, invent a connection, and dream up a problem to solve involving the connection. His example is: "an old blind woman spinning in the Western Highlands, a barn in a Norwegian saeter, and a little curiosity shop in North London kept by a Jew with a dyed beard."
In the real world ugly international doings are afoot and eventually they intrude into Hannay's household. Members of the families of three great men -- "the daughter of the richest man in the world, the heir of our greatest dukedom, the only child of a national hero" -- have been kidnapped and are being held as hostages by a combine whose members are outwardly respectable but which is using the disaffected across Europe and elsewhere to further their own concerns, including fraud, profiteering, and even murder. Though known to the authorities, if members of this combine are captured too soon the hostages will doubtless be executed and certain delicate political matters in the balance upset.
By a twist of fate Dr Greenslade's literary example serves to aid Hannay and his friends get on the track of the villains in a race against a deadline at which they can only guess.
My verdict: Was Buchan following Dr Greenslade's advice, I wonder? Hannay has quite a puzzle to solve and the first half of the book follows his attempts to make sense of the sole clue: six lines of doggerel sent to each of the three great men. There is more intelligence work and less physical action in this novel and the slow working-out of the mystery is convincing. Mary Hannay, while mostly off the page, plays a role near the end that is both gripping and believable and received a loud hurrah from here!