First, an explanation: The Flood in question is not of the biblical sort. Rather it is a “waterless” deluge, an airborne tsunami of germs that kills its frothing, bleeding hosts within days.

Only those lucky few who remain isolated from contamination and avoid contact with the corpses of its victims stand any chance of surviving. Now all they need is to find food, fight off hungry hybrid animals and – worse – the few remaining, marauding and homicidal packs of humans prowling the land.

Atwood does not specify what the year is, though it’s somewhere in the near future. Her novel takes us back through the quarter century leading up to this apocalypse and, unlike many who use that word casually, her apocalypse is truly devastating. It is to the 21st century what the original flood was to Noah, though in many ways far more terrifying. What it leaves in its wake is a world not merely cleansed of almost all polluting humans, but peopled by alarming man-made hybrids – human as well as animal.

Secondly, a warning. The Year of the Flood is the partner novel to Oryx And Crake, Atwood’s previous vision of ecological nightmare. It is marginally less bleak than that vision of human folly taken to extremes, but it’s the kind of margin so slight you need an electron microscope to spot it. At its heart are God’s Gardeners, a renegade, illegal outfit of latter-day hippies who love nature, God and all things natural. In one of the hymns dotted throughout the book, they chant: “God gave unto the Animals/A wisdom past our power to see:/ Each knows innately how to live,/ Which we must learn laboriously...”

In their view, animals are equal with humans (but probably smarter). The Gardeners religiously recycle their goods, eschew meat, fish and even coffee. The worst insult they can hurl is “meat-breath”, and any figure of speech involving an animal – “no room to swing a cat”, for instance – is swiftly followed by an apology, “no offence to cats”. Led by Adam One, and a platoon of Eves and Adams who are the senior members of the group, this is a loveable, kind, shrewd band of eco warriors, at odds with the rampantly consumerist and insanely experimental world they have foresworn.

Atwood’s horrifying world is dominated by CorpSeCorps, a military outfit who oversee law and order with an iron fist and, when that fails, with bullets. Those in their employ live in wealthy compounds; those beyond its safety run feral in the pleeblands. Both parties are only slightly exaggerated versions of today’s ghettoes; of the sickeningly rich, who spend their money on perfecting their bodies, and the urban poor, who are scavengers and street fighters. By comparison, God’s Gardeners, squabbly and odorous as they are, appear almost saintly in their altruism and pacifism.

The pleeblands, the compounds and the whole miserably apartheid scenario is already familiar from Oryx And Crake. What we find in The Year Of The Flood, however, is the backstory to the devastation that dominates both books. One of the narrators who tells us how things have reached this state is Toby, formerly a loyal but reluctant member of God’s Gardeners. After the flood, she finds herself alone in a beauty salon – AnooYoo – where she was once manager. Against the inevitable plague they all predicted was on its way, she has squirreled stores of food, but these are running low and she must soon consider going out foraging, dangerous though that will be.

Also isolated, but in her case locked into a room she cannot escape, is Ren, a sex worker who was a child when Toby first knew her. As Atwood spins out the story of these unfortunate women, the threads that bind them to the first novel slowly emerge. There are plenty of other tales and characters too, almost unvaryingly dreadful. Indeed, as the pieces of Toby and Ren’s world begin to come together, and the plot advances from the past into the deadly present, the relentless nastiness of her heroines’ past lives is sobering. For all that, Atwood maintains an almost sprightly tone, using flashes of dry wit to punctuate what might be otherwise unremitting gloom.

The intention might be good, but it creates a tension that undermines her material, emasculating the fear her setting should evoke, and creating a mood more like that of a novel aimed at teenagers than at the mature and devoted readers who eat up Atwood’s every word.

With her familiar device of using women’s voices to drive the story, Atwood explores her favourite themes: the female body and its abuse, women’s friendships, the vileness of men and ecological prophecy. While they are passionately felt, none of these strands is handled with subtlety. In fact, using prosaic, at times clumsy language, Atwood hammers her themes so hard, the effect is numbing. Though her plot is dramatic, and her points important, The Year Of The Flood is tough going, a dark message strung out over too many pages, with too little artistry to illuminate the journey.

The Year Of The Flood by Margaret Atwood, Bloomsbury, £18.99.