Household writes in the style and tone of John Buchan. Their heroes are gentlemen, ex-officers with war and espionage experience, now retired, with time on their hands. They have careers (Charles Dennim in this novel is a zoologist studying squirrels), or maybe just hobbies, but they don't ever seem to have bosses. When the lure of being either an assassin or a quarry looms, they drop everything to eagerly take part.
The problem is that this style and tone is very antique for the book's setting of 1955. Dennim lives in suburban London with his Aunt Georgi, whose "conversation, like that of many intelligent women, only made sense retrospectively. I mean that it appeared incoherent until it arrived at its destination - when all the rest, if you could remember it, fell into place and was relevant." England is his adopted country; he's really an Austrian Count - the Graf von Dennim. During World War II he served the British undercover in the Gestapo at Buchenwald, and turned down the George Cross, feeling his actions would dishonor it. Now, someone unhappy with his Buchenwald role is trying to kill him - a parcel explodes at his front door, killing the postman.
Dennim's solution is not to turn over the matter to the police, and obtain a bodyguard, but to become bait, to become the goat tied to a stake that lures the tiger. The tiger metaphor is exhaustively, tediously paraded. He borrows a cottage in Buckinghamshire where he can pretend to observe badger setts. The assassin follows. Dennim befriends an assortment of countryfolk and survives some poisoned veal cutlets. He moves to the Cotswolds, borrows an Arabian stallion, develops a crush on a vicar's daughter 20 years younger, and meets his assassin. They sit down for a whiskey at an inn and come to an agreement over how the next part of the battle will be fought, as if this were an 18th century duel.
You're right, this doesn't make a lot of sense.
Then they retreat, each with his horse and revolver, to their respective hideouts a few hundred yards apart, and wait for nightfall. There follows a lot of stumbling around in a barn and a gunfight in the dark.