Those of us lucky enough to read The Handmaid’s Tale back when it first appeared in 1985 will remember the shock of a novel that felt both claustrophobically precise and shatteringly prescient. The newly born Republic of Gilead, with its abuses and abominations, its hideously misogynistic vocabulary and gruesomely rationalised constraints, was just about far enough from our own world to seem beguiling, but also close enough to feel like a wake-up call.

Still, no one could have guessed the extent to which recent history (as well as a superb TV offshoot) would bring it eerily, terrifyingly back into focus. With the implacable rise of the Christian right in the US, never has it felt more urgent for women to guard both their bodies and their reproductive systems against (some) men and the state. Who, after all, is Donald Trump if not Commander Waterford without the charm? And I can’t be the only one who, watching the Trump crowd’s chants of “Lock her up!” or “Send her back!”, found herself thinking of taser-wielding Aunt Lydia and the handmaids’ cries of “Her fault!” when one of their number “confesses” to having been gang-raped?

Given all of this history – and the fact that for 35 years fans have apparently been begging for answers to a host of Gilead-related questions – it’s not surprising that The Testaments feels as eagerly awaited as a handmaid’s bouncing baby. If ever a novelist could justify the spawning of a sequel, Atwood can.

This novel opens 15 years after the end of the last book. Perhaps wisely, Atwood neatly leapfrogs the TV series – presumably leaving it open for several more seasons – while plucking just one significant detail from it: Baby Nicole. The “child” of Commander Waterford and his wife (though in fact, as we all know, born of Offred/June and fathered by her lover, Nick), Nicole was smuggled into Canada as a baby and has not been seen since.

This – together with the constant “seepage” of handmaids being helped to freedom by the so-called Underground Femaleroad – has not been good for Gilead, which has settled into an inevitable “dog-eat-dog maturity”. Meanwhile, a great deal of effort and espionage are going into tracking down missing Baby Nicole – now seen as the veritable “poster child for Gilead” – so that the republic can reclaim her as their own.

The novel is narrated by three different female voices. The first belongs to Aunt Lydia herself, who is secretly writing her memoirs, apparently largely “for your edification, my unknown reader”. The second is Agnes, a young woman who has grown up in Gilead and is being groomed (in both senses of the word) to marry a commander. The third, Daisy, is a feisty teenager living in Canada with two people who run a thrift store and whom she supposes to be her parents, except that something has always felt wrong: “It was like I was a prize cat they were cat-sitting.”

How these young women may or may not be connected – to one another and also, in Daisy’s case, to Gilead – is one of the questions that drives the first hundred pages or so of the novel. Certainly, right from the start, Daisy lets slip that she discovered on her birthday that she was a “fraud – a forgery done on purpose”. And Agnes, who used to think she remembered nothing before the age of six or seven, realises she does have one “hazy memory of running through a forest with someone holding my hand”. Aunt Lydia, meanwhile, who is “everywhere and nowhere”, knows everything that they (and we) don’t know. Atwood has always said that “knowledge is power”, but will Aunt Lydia use her power for good or evil?

Atwood challenges us to think and rethink, to see our volatile world anew

What follows is a plump, pacy, witty and tightly plotted page-turner that transports us straight back to the dark heart of Gilead and seems to take great pleasure in providing answers to many of Atwood’s readers’ questions. If, for instance, you ever wondered why Aunt Lydia seems so willing to join the regime’s monstrous attack on her own gender (despite a queasily hinted at softer side), here’s her backstory. Similarly, the inner workings of Gilead are exposed in far more detail than in the first book and it’s hardly surprising to learn that “beneath its outer show of virtue and purity” the place is “rotting”.

What is surprising, though, given that so many of Atwood’s actual details remain so gloriously dark (a paedophile dentist whose hand sits on a pubescent child’s breast “like a large hot crab” is an image that won’t leave me in a hurry), is that the story’s outcomes are anything but. Perhaps Atwood has simply decided that Gilead’s time is up – or maybe she’s grown too fond of her characters to deny them happiness – but in so many startling ways this novel feels like a straight antidote to The Handmaid’s Tale.

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Where the first book traded so pithily and memorably in obfuscation, despair and darkness, the sequel sees the lamps slowly lit. Where, at the end of the first, June was bundled into the back of a van with no idea whether it heralded her “end” or her “beginning”, in this second novel we have a quasi-Shakespearean sense of all’s well that ends well. Not only do various bad guys receive their comeuppance, but there’s a strong sense of goodness winning the day, even – whisper it – hints of something that might amount to a happy ending.

Which actually feels a touch disappointing. Sure, this new book is gloriously savage in its anger, against both God, with his “special interest” in the “polluting” blood that comes out of girls, and sexually predatory men who prefer their underage victims to seem “not fully human, with a naughty core to them”. There is no doubt that Atwood is on top form here. But still it feels as if something crucial is missing.

Or perhaps not missing enough, for didn’t the strength of the first book lie precisely in its daring ambiguities, its unapologetic refusal to elucidate? Surely one of the reasons Gilead managed to be so spookily convincing was that Atwood cunningly chose to leave so many of its edges blurry. Interiors, furniture, food, clothes, linen were described with all of the deft shadow and gleam of a Dutch painting – and the same, incidentally, is true in this book – but beyond that, we only had the vaguest hints of how the larger world worked. The most trenchant and exciting fiction almost always amounts to an act of conjuring – and in Atwood’s gracefully necromantic hands, Gilead’s regime didn’t seem to require much explanation or justification. We believed, simple as that. And as for why Aunt Lydia behaves as she does? Well, have any of the world’s most brutal regimes ever run short of compliant executioners?

Another problem, which becomes more troubling as the novel unfolds, is the lack of emotional subtext, or indeed sometimes any subtext at all. In The Testaments, what you see is what you get, with any possibility of equivocation, shading or real complexity (or the chance for readers to imagine anything for themselves) sacrificed again and again to pace and plot. Perhaps the emphatically retrospective nature of this narrative doesn’t help. The Handmaid’s Tale unfolded in a memorably deadpan and very immediate present tense – we didn’t know what was coming next any more than June did. But here, because all three central characters are apparently giving their accounts as “witnesses”, events unwind in the far less suspenseful and all-too hindsight-laden past.

Perhaps because of this, there are few, if any, chances to feel moved on behalf of these characters – a strange and enervating absence in a novel that hinges so strongly on the agonies of familial separation. It made me wonder, not for the first time, whether this simply isn’t where Atwood’s interests lie: the only scene in The Handmaid’s Tale that, on a recent rereading, seemed less than entirely convincing is when June, finally shown a photograph of her daughter and unable to find any trace of her own existence in her child’s eyes, wishes she hadn’t seen it at all. (Believe me, no mother on Earth would pass up on that photo.)

Where Atwood’s interests do undeniably lie is in shaking us up, challenging our complacencies and using her chillingly profound imagination to challenge us to think and rethink, to see our volatile and increasingly toxic world anew. But is she willing to leave room for her reader? I have my own test of what makes a truly great work of fiction: can you revisit it at a later point in your life and read a whole different novel? In other words, is the novel sufficiently elastic – and slippery and enigmatic – to grow with you?

The Handmaid’s Tale triumphantly passes this test. But occasionally, with its wide-angle sweep and wholehearted lack of uncertainty, its angels and demons struggle and seemingly effortless resolutions, The Testaments can feel as if it’s already decided what it thinks. And what we should think, too.

Julie Myerson’s most recent novel is The Stopped Heart (Jonathan Cape).

• The Testaments by Margaret Atwood is published by Chatto & Windus (£20). To order a copy go to guardianbookshop.com or call 0330 333 6846. Free UK p&p over £15, online orders only. Phone orders min p&p of £1.99