Margaret Atwood had a cameo in the television series based on her book The Handmaid’s Tale. She played an Aunt in a scene where a woman is ritually shamed by a group of handmaids for “getting herself” gang raped at the age of 14. “Her fault, she led them on,” is the chant they use. Atwood says she found the scene “horribly upsetting”, although it was possibly not so wrenching to write as it was to enact or, later, for us to watch.

In the original book, a few deft sentences lead the reader, not into the magnetising shaming of another human being, but to the narrator Offred’s insight into her own complicity. “I used to think well of myself,” she says. “I didn’t then.” The scene is moral, not sensational; it works through the brain, not through the eyes. This is one reason Atwood’s work feels so ageless and necessary. She thinks.

Atwood certainly has had an enormous amount to think about since her novel went supernova, not just as the hugely successful television adaptation, but as a powerful symbol of resistance to the misogyny of Donald Trump and the Christian rightwing. The series became a kind of visual enlargement of the agonies of the age, or the female agonies at least. It was sometimes hard to look, or to look away.

In The Testaments, Atwood reclaims the right to consider such difficulties rather than simply imagine them. She is interested not in how people become degraded, as objects (that is so easily done), but how they became morally compromised.

The novel picks up 15 or 16 years after Offred disappears to an unknown fate at the end of The Handmaid’s Tale. There are three narrators, two of them young and idealistic, one of them old and endlessly cunning. The most compelling portrait is that of wickedness – of course it is. The story is driven and described by the infamous Aunt Lydia, and she is just as terrifying, in her astringency, as you would expect her to be.

In Lydia’s world view, people rise and fall by strength or weakness, and justice is a kind of theatre. “Innocent men denying their guilt sound exactly like guilty men, as I am sure you have noticed, my reader.” She appeals to the heartless survivor in all of us – at least this is what she seems to say, that when the chips are down, we will revert to our most primitive state. A crowd of imprisoned women is described as “crocodiles”, ready to “leap, thrash about and snap”. Their first sighting of a mass execution does not dull their appetite for food, in fact it does the opposite: afterwards, Lydia is given an egg sandwich and, “I am ashamed to say, I gobbled it up with relish”.

To read this book is to feel the world turn­ing, as the unfore­seeable shifts of recent years reveal the same themes

Her induction into the order of Aunts is described with a chilling vigour. Tortured, imprisoned and tested, she is given a choice, and she chooses “the path most travelled by”, one of compromise, betrayal and lies. The first book was good on the envy between women, when they have no power; The Testaments looks at collaboration – another vice of the oppressed. Lydia, however, collaborates as an equal, not as a victim; she is not in thrall. Indeed, she is happy to destroy women who have internalised the values of the patriarchal regime: one girl, Shunammite, is coldly sacrificed to her own silliness, a move that Lydia seems to enjoy.

Lydia may be in charge of the novel, but hers is not the only point of view. The younger women, introduced as Agnes and Daisy, have not lived a life of compromise, they do not remember the old days and they betray no one. Daisy is as bolshy and idealistic as any other teenager, Agnes is loyal and ready to love. The girls are courageous and hopeful and they venture out in the second half of the novel like the heroes of old. They believe in friendship: they even witness the perseverance, after death, of a person’s soul.

Facebook Twitter Pinterest Aunt Lydia, right, in the TV adaptation of The Handmaid’s Tale. Photograph: Sophie Giraud/Hulu

The dystopia of Gilead has worn very well, even though its putative future date now lies in our collective past. People use computers, but the internet, that constant plot spoiler, has been more or less suppressed, and with it, the tendency of history to spin off into unlikely or cruel absurdities. Trump could not exist in this enclosed universe. Gilead was first imagined by Atwood after living in West Berlin before the fall of the wall. It is a place where people do not know things, including the sins of those in charge. Information is used at the end of the book as a liberating force rather than a randomising one. Authority exists, it has been centralised, and it can be overthrown.

In The Handmaid’s Tale, Offred refers, with amazing prescience, to “false news”: in The Testaments the news has turned “fake”. The word “slut” is more frequently employed, but otherwise there is no need to change what was, in 1985, so properly realised. As Atwood says, everything in this future dystopia has happened somewhere already, and Gilead owes much to the US’s Puritan past. She is strong on idleness and good manners as weapons of oppression; for Atwood, perhaps the worst possible fate in life is to have nothing to do all day.

As a novel, The Handmaid’s Tale is held together by the sexual tensions between the characters. It is, among other things, a claustrophobic book about adultery, or one in which adultery has been turned to ritualised rape. The characters in The Testaments do not yearn and mourn as Offred did. They have, at the beginning of the book, been scattered by happenstance. This open, plot-driven novel brings them together in a way that owes less to Pinter’s Betrayal, and more to a Shakespearean comedy of children lost and found.

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In all this losing and finding, Aunt Lydia behaves as the novelist does, bringing people together for the purpose of plot, which in her case is also plotting. The novel is her conspiracy of one.

Some Gilead facts invented by the readers make it into The Testaments. “June” was chosen by fans as Offred’s given name, and Atwood is happy to accept the suggestion. “Nicole”, a baby who first appears in the TV series is, in The Testaments, nearly fully grown.

You might call this interleaving of book and reader postmodern, but there is more here than a posh writer’s punning. Gilead, the fiction, is a kind of overgrown child. Atwood has taken it by the hand and made an open, free-running story, one that remains, as ever, deeply informed. In writing The Testaments, she also reclaims its world from all the people who think they own it now: the writers of fanfiction and the television producers (she told them they could not kill Lydia, apparently). A story that feels universal is, actually, hers: she gets to decide.

Perhaps no other writer has managed her own phenomenon with so much grace and skill. The Testaments is Atwood at her best, in its mixture of generosity, insight and control. The prose is adroit, direct, beautifully turned. All over the reading world, the history books are being opened to the next blank page and Atwood’s name is written at the top of it. To read this book is to feel the world turning, as the unforeseeable shifts of the last few years reveal the same old themes. It is also a chance to see your own political life flash in front of your eyes, to remember how the world was 30 years ago and say: “If she was right in 1985, she is more right today.”

• Anne Enright’s The Green Road is published by Vintage. The Testaments by Margaret Atwood is published by Chatto (£20). To order a copy go to guardianbookshop.com. Free UK p&p on all online orders over £15.