by J. K. Rowling
Several months after solving the Lula Landry case, Cormoran Strike is tasked by Leonora Quine with locating her novelist husband Owen, a former literary genius whose attempts to recreate his past success have failed. Owen disappeared around the same time his latest book, Bombyx Mori, was leaked. The book has been deemed unpublishable due to its mixture of sexual assault, torture, and cannibalism as well as its slanderous depiction of the people in Owen's life. In addition to Leonora, Strike sets out interviewing the other people portrayed in the manuscript: Owen's lover Kathryn Kent, protégée Pippa Midgley, agent Elizabeth Tassel, editor Jerry Waldegrave, publisher Daniel Chard and former friend Michael Fancourt. The suspects, however, soon turn on one another, accusing and counter-accusing each other of killing Owen and ghostwriting Bombyx Mori.
As the investigation commences, Strike's relationship with Robin Ellacott gradually deteriorates, as she feels neglected by him and he feels unwilling to put her in a position where she is forced to choose between her job and her fiancé Matthew. The animosity is tempered when Strike finds Owen's body, which has been mutilated, doused in acid and posed to resemble the ending of Bombyx Mori. Metropolitan Police later arrest Leonora for the murder, prompting Strike to set out clearing her name. Robin, meanwhile, strains her relationship with Matthew after she almost misses his mother's funeral to help Strike and gets caught telling a lie. She later confronts Strike about his intentions only to be warned that she will be asked to do things Matthew will not like if she becomes an investigator.
With the case against Leonora piling up, Strike focuses on Fancourt, whose character in the manuscript is inconsistent with his relationship to Owen. Several years earlier, after Fancourt's wife wrote a novel that was panned by critics, an anonymous parody's release prompted her to kill herself. Fancourt accused Owen of authoring the parody and Tassel of enabling him. Strike soon deduces Bombyx Mori is a metaphor for someone else's life and Owen was intended to be the antagonist rather than the hero. Realizing the manuscript was penned by a ghostwriter, he creates a plan to confront the killer. He later approaches Fancourt at a party and asks to speak to him in private. When Tassel, who is also in attendance, joins them, Strike accuses Tassel of being Owen's killer and the ghostwriter.
Tassel, a failed author herself, wrote the parody of Elspeth's novel, which Owen used to blackmail her for twenty years. When he approached her with the original concept for Bombyx Mori, Tassel concocted an elaborate plan. She conspired with Owen to stage his disappearance, rewrote Bombyx Mori, killed Owen and framed Leonora. Tassel attempts to flee, only to be caught and arrested, which Strike and Robin planned in advance. Sometime later, Leonora is released from prison, Fancourt acknowledges the original Bombyx Mori manuscript's literary value, and Strike tells Robin that he enrolled her in investigative training courses as a Christmas gift.