It isn’t too much of a spoiler, I hope, to say that Robert Harris’s enjoyable new book has a twist not at the end, but at the beginning: it starts out looking like a historical novel and, a chapter or two later, turns out to be science fiction. The opening gives it the whole nine yards:

Late on the afternoon of Tuesday the ninth of April in the Year of Our Risen Lord 1468, a solitary traveller was to be observed picking his way on horseback across the wild moorland of that ancient region of south-western England known since Saxon times as Wessex. If this young man’s expression was troubled, we may grant he had good cause. More than an hour had elapsed since he had last seen a living soul. Soon it would be dusk, and if he was caught out of doors after curfew he risked a night in jail.

The slightly camp olde-worlde locutions would seem to locate the narration somewhere in the 19th century, though the date locates the action in the 15th century. We settle in. The protagonist, Christopher Fairfax, is a youngish priest journeying through a wild and woolly late-medieval landscape – all menacing weather effects, raven-haunted gibbets and scrofulous, God-fearing locals. He is, we discover, en route from Exeter, seat of the bishop, to the rural parish of Addicott St George to conduct the funeral of a long-serving parish priest.

When Fairfax arrives at his destination, we come face to face with Father Lacey’s body, slightly ripe by now, laid out in a coffin in the parsonage. There’s a mildly dour housekeeper, a comely but mute niece, tallow candles and homely vittles. But there’s something slightly odd about the deceased: Father Lacey, unlike most clerics, was clean-shaven. There are dark allusions to his eccentricities and a suggestion that his death in a fall near the Devil’s Chair (‘a lonely, fearsome spot’) may not have been an accident. In Lacey’s study, Fairfax discovers a remarkable private library that holds twenty volumes of The Proceedings and Papers of the Society of Antiquaries, books that are clear signs of heresy on a capital scale: ‘The organisation had been declared heretical, its officers imprisoned, its publications confiscated and publicly burned, the very word “antiquarian” forbidden from use.’

What’s more, Father Lacey owned a collection of knick-knacks of a sort the Church would definitely have frowned on. Pick of the bunch is ‘one of the devices used by the ancients to communicate’:

He pressed the button on the front, as if it might miraculously spring to life, but the glossy surface remained resolutely black and dead, and all he could see was the reflection of his own face, ghostly in the candlelight. He turned it over. On the back was the ultimate symbol of the ancients’ hubris and blasphemy – an apple with a bite taken out of it.

And, boom, we’re off to the races. This is not medieval England after all; it’s some dreadful post-Brexit scenario in which, a millennium or two after the end of our civilisation, the surviving dregs of humankind have blamed the catastrophe that caused it on divine retribution and turned to the grim comforts of the Church. The calendar has been reset, the only vocabulary allowed is that which appears in the King James Bible and any curiosity about the good old days is strictly verboten.

So the story of Father Fairfax’s time in Addicott St George is one of a closed mind creaking none too reluctantly open, and the investigation of a possible murder. For the reader, there is also the desire – never wholly satisfied – to discover exactly what it was that did for 21st-century life: nuclear holocaust, climate change, sunspots, a pandemic virus, a pandemic computer virus or any other of the usual suspects.

We learn that the furtive antiquaries of the future have pieced some of it together. They know that we had air travel and they puzzle over what ‘The Cloud’ may have been. Motorised wagons, buried deep in the loam, appear only as smears of rust. A few stone and concrete structures, such as ancient churches and motorway cuttings, survive in the landscape for the moderns to puzzle over, as Anglo-Saxon poets puzzled over the ruined Roman structures left by, as they supposed, ‘giants’. The second sleep of the title, then, isn’t just a reference to the two-part night-time routine of medieval people but also to a second sleep of reason.

This is all very readable and intriguing, and Harris sets a number of hares running. What’s up with the lady of the manor and her hulking, saturnine suitor Captain Hancock? Is there, as rumoured, hidden treasure buried somewhere underneath the Devil’s Chair? And is Fairfax going to get into trouble with his worldly and politicking bishop? The novel suffers a little, I think, from the sheer number of hares set loose though: the world-building isn’t completely convincing, the plot is definitely on the harum-scarum side and the conclusion makes you wonder if Harris hasn’t boxed himself just slightly into a corner.

But speaking as a sucker for the Riddley Walker/The Book of Dave/Planet of the Apes school of post-apocalyptic stories, I liked it very much. It’s an involving entertainment containing some salutary ideas about how any theocracy leans on ignorance and fear, about the fragility of our own technological utopia and about the way history is just as likely to move in circles or gyres as in a straight line.