The Ink Black Heart

After Strike and Robin visit The Ritz for Robin's 30th birthday, Strike attempts to kiss Robin; she evades the kiss. Feeling rebuffed, Strike starts a relationship with Madeline, an acquaintance of his ex-fiancée Charlotte, a relationship he keeps secret from Robin.

Edie Ledwell, an animator who co-created the successful cartoon The Ink Black Heart on YouTube and which is now being adapted into a film on Netflix, visits the agency. She asks Robin to investigate the identity of Anomie, an online figure who co-created Drek's Game, an online game based on the cartoon, and started harassing Edie after she criticised the game. Robin refers Edie to another agency with more cybercrime experience. Within the game, two moderators appear to have a dossier of proof that Anomie and Edie are the same. They share this with Josh Blay, the other co-creator of The Ink Black Heart and Edie's ex-boyfriend. Soon afterwards, Edie and Josh are tasered and stabbed while meeting in Highgate Cemetery, the cartoon's setting. Edie dies while Josh is paralysed.

The agency is hired to investigate Anomie's identity by a film producer seeking to adapt The Ink Black Heart. They investigate various individuals associated with the cartoon and the North Grove Art Collective. Much of the investigation takes place online with the detectives investigating Anomie's abuse and another figure, The Pen of Justice, who criticised the cartoon for being racist, ableist and transphobic. They also investigate Drek's Game, where Anomie openly confesses to the murder, something treated as a joke by the other moderators, including its co-creator Morehouse. Two moderators appear to be associated with the Halvening, the far-right group that compiled the dossier with fake proof and the police suspect committed the murder. Robin accesses the game and becomes an active player. Robin and Strike attempt to eliminate suspects by carrying out surveillance and examining who is otherwise engaged while Anomie is active in the game. They also receive phone calls telling them to exhume Edie's grave and open letters buried with her. In the game, Paperwhite, another moderator, and Morehouse appear to have a relationship, with Paperwhite sending a racy picture to Morehouse and including Anomie by accident.

After leaving Comic Con where Robin interviewed Yasmin, the former employee of Edie and Josh, they follow a suspicious individual, only for a man dressed as Batman to push him onto tracks as a train approaches. After Robin helps save his life, her photograph appears in the newspapers. It is revealed that she saved Oliver Peach, the moderator Vilepechora in Drek's Game and member of the Halvening. In the game, Anomie confesses this crime to Oliver's brother, LordDrek, before banning him from the game. Soon afterwards, a parcel bomb damages the office, although nobody is injured. The publicity causes Morehouse to discuss going to the agency with Paperwhite. Robin is able to speak to the moderator Fiendy1 on a personal level and discovers she is Edie's cousin. She is able to provide the identity of Morehouse and Strike and Robin decide to interview him, but Morehouse is murdered before they reach him.

Strike interviews Yasmin himself and discovers that she was being blackmailed by Anomie to log in as them on several occasions, rendering much of their work to eliminate suspects moot. Robin also receives the picture that was supposedly sent by Paperwhite to Anomie and is able to trace the girl to a Glasgow art student. They then figure out Paperwhite was a sock puppet account controlled by Anomie to keep tabs on Morehouse and the other moderators. She then receives a phone call, threatening to kill her. Strike realises that Edie's uncle did not bury Josh's letter with Edie. After reading misogynistic abuse in the letter, they deduce someone with access to Katya, Josh's agent, replaced the original letter.

Soon afterwards, Katya's daughter calls them, screaming for help. After driving to Katya's house, Gus, Katya's son, now revealed as Anomie, tasers Strike. Robin sets off a rape alarm before fleeing upstairs, where she sees Gus's father's corpse. A machete-wielding Gus pursues her until he is distracted by neighbours alerted by Robin's alarm, allowing her to hit him in the back of the head.

In hospital afterwards, Strike tells Robin that her name has been added to the office door, which brings her to tears, and that he has broken up with Madeline. Robin, who believed he was still dating Madeline, reveals she is now dating the officer, Ryan Murphy. After she leaves, Strike reflects that he may have missed his chance to date Robin.

The Ink Black Heart sold 50,738 copies in its first week on sale in the UK, placing it first on the UK Official Top 50 book sales list.[3]

Jake Kerridge from The Daily Telegraph rated the book 3 out of 5 stars, describing the series as a whole as "good comforting crime fiction", but criticising The Ink Black Heart for its length, stating it "[does not] seem to have more depth, or to cover more emotional territory, than the earlier ones did".[2] The author Mark Sanderson, writing in The Times, similarly criticised the length.[4]

Kirkus Reviews called the book "[a]n overblown whodunit", citing length and extensive focus on online conversations as reasons to skip it. They concluded the review by saying "[a]fter a thousand pages ... the reader is likely to no longer care" who the murderer is.[5] Darragh McManus from Irish Independent gave the book a positive review, praising it for "dozens of characters, multiple plotlines and, most crucially, lots and lots of things going on".[6]

In a review for the Los Angeles Times, Bethanne Patrick called the novel “a crashing bore” and criticized its stereotypical characters, concluding that “[p]erhaps Rowling should […] divorce her writing from murder mysteries, and dig deep instead into the things that really matter to her: women (or at least cisgender women) and children.”[7]

The plot, in which a woman is killed after being accused of various prejudices, particularly transphobia,[13] drew comparisons to Rowling's real-life previous controversial comments surrounding transgender people, leading to allegations of self-insertion.[14] Rowling denied the claims that the book was inspired by her own controversies, stating, "I had written the book before certain things happened to me online".[15][16][17]

The book was shortlisted for the 2023 CWA Ian Fleming Steel Dagger.[18]