Although most often billed as a sci-fi writer, M. John Harrison’s work is far too various for easy categorisation, encompassing as it does the satirical space opera of The Centauri Device, the urban fantasy of the Viriconium sequence and the more conventional mountaineering novel Climbers.

The Sunken Land Begin to Rise Again, his first novel since 2012, might seem similarly conventional on the surface, with its contemporary setting, its mild and approachable characters, but beneath this veneer of normality lurks one of the strangest and most unsettling novels of the year.

The book follows two characters equally submerged in directionless, low-key midlife crises or breakdowns. Shaw, in his fifties, finds himself renting a room in a boarding house in south west London. He starts a casual relationship with Victoria, whose mother has recently died and left her a large house in Shropshire.

Shaw finds an IT job working for Tim, who has an office on a houseboat in Barnes where he curates The Water House, a cryptic conspiracy theory website, and from where he self-publishes an obscure scientific tract called ‘The Journey of Our Genes’. Tim sends Shaw on strange assignments around the country; to report on a bizarre court case where the defendant claims to have seen a green, childlike creature “which possessed the qualities of both a foetus and a fully formed organism” in a pub toilet; or to visit a medium who falls into a trance when she holds Shaw’s hand.

At the same time, in Shropshire, Victoria finds her new surroundings unsettling; strange figures are glimpsed on the edges of local woodland near the Severn river; disembodied voices call incomprehensibly from nearby streets; workmen and chance acquaintances make gnomic statements and press battered copies of Charles Kingsley’s twee Victorian morality tale The Water Babies into her hands. She also strikes up a relationship with a cafe-owner who she sees disappear into a pool.

In an affectless, disordered way, both Victoria and Shaw find themselves tangentially connected to an inexplicable and never elucidated conspiracy about human mutation or hyper-evolution, or the possible existence of an entirely separate aquatic human species. At one point, as Shaw, baffled, leafs through a print-out of material from the Water House website, Tim excitedly notes connections that seem to have no causal relationship. ‘“In the end,”’ Tim says, ‘“Is logic in any sense the right method to be applying here?”’

As all this might demonstrate, this is a novel of such weird and brooding opacity that it’s almost hard to describe what it’s actually about in the first place. It has all the lucid certainty of a dream, and I don’t think I’ve ever read something shot through with such a minatory sense of dread and unease. As Tim acknowledges though, logic and causality are not the key to unlocking this novel’s meaning, and Harrison’s real achievement here is in holding out the possibility of an explanation that always seems to slither away at the last moment.

As Shaw and Victoria stumble in their apathetic way through the drowned ruins of their respective lives, they’re similarly on the cusp of an elusive understanding. Neither character feels compelled to chase these mysteries down though, if mysteries they truly are, and it’s this sense of meditative rather than reactive disquiet that gives the book its real force and power.

If fiction’s greatest achievement is in affecting the way the reader sees the world, then Harrison’s spare and beautiful prose has conjured up here a feeling of almost permanent dislocation from the routines of everyday life. Even when the book has been closed, it’s a feeling that is incredibly hard to shake off.