In Oxford's Bodleian library, brilliant American historian and reluctant witch Diana Bishop unwittingly calls up an ancient magical manuscript, Ashmole 782, which has been missing for centuries, and finds herself confronted by acclaimed geneticist and vampire Matthew Clairmont, who is determined to get his hands on the book. Diana's determination not to be frightened by Matthew as he struggles to control his predatory instincts around her fuels a dangerous connection between the two of them. When news of the book's appearance spreads throughout the world of daemons, vampires and witches hiding in plain sight among humans, the hunt for the witch who summoned it takes on a deadly intensity.
Blind to the interest she has stirred among creatures, Diana fights an increasing pull towards the magical and turns her back on her witch heritage, but matters take an unexpectedly dark turn when Satu is sent by Peter Knox to intimidate her. In Venice, beautiful but murderous Juliette has her fatal obsession with Matthew reawakened. And just as Diana decides to trust Matthew with a secret only she knows about the book, he flees in a desperate bid to protect her from his cravings. But in so doing, he leaves Diana exposed to all her enemies.
Domenico takes information he's extracted from Satu to menacing vampire Gerbert, who gains the advantage he has been waiting for. After sending Juliette into the world to do his bidding, Gerbert consults with Meridiana, the witch whom he keeps in thrall, but what does the prophecy she delivers mean? In Oxford, Diana refuses to help Knox realise his supremacist longing to destroy the vampires and answers his threats with such a violent force of untamed elemental power, as to set in motion a wider chain of events that bring her closer to Matthew but place her in even greater jeopardy.
Matthew's hope of keeping Diana safe at Sept-Tours appears short-lived as Ysabeau refuses to welcome the warmblood under her roof. Knox calls an emergency meeting of the Congregation to denounce Matthew for abducting a witch but Matthew's ferocious older brother Baldwin plays a careful game to maintain control in the face of Gerbert foretelling the end of the de Clermonts – and throws Diana to the lions. Diana isn't the only witch facing persecution. Satu pays a shocking price for her curiosity when Knox discovers she has been searching through the Witches' Archive and learned his guilty secret. Meanwhile, things turn nasty in Oxford for Marcus and Miriam as Juliette attacks.
Gillian pays a heavy price for her disloyalty to Diana, but not before news that vampires are studying witch DNA reaches Knox. The fall-out threatens the fragile equilibrium between creatures and the focus is on ending Matthew and Diana's relationship whatever the cost. Agatha treads a careful line of democracy at the Congregation, but Nathaniel continues to instigate daemon direct-action. Ysabeau relies on shock tactics to expel Diana until Matthew returns with breathtaking news about Diana's DNA. Diana and Matthew are finally united in love, only for the unexpected to happen.
Satu brutally tortures Diana in Gerbert's isolated castle ruins. Matthew, now sworn to hunt down and kill anyone who tries to harm Diana, attempts a daring airborne rescue mission with Baldwin in tow, but they are too late to prevent Satu invoking dark magic in a bid to open up Diana and see the magic inside her. As Diana is forced to confront the idea that she might have been spellbound, she wonders what kind of monster she is. When Sophie confesses that she is a daemon born of witches, this news of cross-species offspring leaves Agatha reeling as she ponders the implications for all their lives.
After confronting her aunts, Diana learns the tragic circumstances surrounding why she was spellbound as a child. Diana's devastation at having lived her life as only half herself is interrupted by the Madison house releasing one of the missing pages from Ashmole 782, sent forward in time by Diana's father Stephen, a time-walker. As their love grows, Matthew confesses to Diana that he wasn't able to keep his two previous loves safe. Diana believes he won't hurt her. But an unexpected encounter threatens to tear them apart once again.
Matthew's attempt to rescue Diana from Juliette goes horribly wrong and Diana finally discovers just how lethal her magical powers can be. As Matthew lies dying, Diana must strike a dreadful bargain with the goddess and make the ultimate sacrificial pact with enduring consequences. A failed coup at the Congregation sees its most powerful and terrifying members go rogue, as they close in fast on Madison, where a group of daemons, vampires and witches are now residing together. Baldwin tries to alert Matthew that time is up but he can't reach him. It's a deadly race against time for Diana to take flight with Matthew before their pursuers arrive.