The crime fiction of Tana French is made of shadows. What matters most is what the detective, usually a member of the (fictional) Dublin Murder Squad, can never know—and because the detective is the narrator, the reader will never know it, either. The killer may be identified, but the truth slips away in the night, as the truth too often does. French’s defiance of crime fiction’s central premise—that all mysteries can be solved—annoys the genre’s least imaginative fans and has made her beloved among its more adventurous devotees.

For shadows to form, however, you need light, a principle that Sarah Phelps—the showrunner of Dublin Murders, a BBC/Starz series based on French’s novels—should have kept in mind. Glum and murky, Dublin Murders makes literal what French only alludes to in pursuit of a Nordic noir melancholy that doesn’t quite gibe with the series’ Irish setting. French’s novels are dappled, not perpetually overcast. Her narrators start out with or remember or luck into golden moments, charmed interludes in which they step out into the full sun, when they believe they have finally found their place in the world. But those idylls are always slipping away, leaving the narrator bereft, pining, or scrabbling to get it all back. Unlike Scandinavian crime fiction, French’s has a lyrical heart, even if that heart has been broken.

Still, Dublin Murders might have made for a pleasingly mopey whodunit along the lines of Shetland or Broadchurch if Phelps weren’t so obviously unsure of her source material. She’s best known in the U.K. for holiday season adaptations of Agatha Christie novels—And Then There Were None, The Witness for the Prosecution, and The A.B.C. Murders, all in the last few years—in which she made such anti-cozy moves as casting John Malkovich as Hercule Poirot. But Christie novels have a sturdy, complex scaffolding of plot tricked out with a full complement of twists, which Phelps updates by adding psychological undercurrents to Christie’s ingenious parlor games.

French’s first novel, In the Woods, by contrast, is notoriously easy to figure out if you think the point is to determine who killed the 12-year-old girl whose corpse is found in a forest at the beginning of the book. But in French’s fiction, such puzzles are only the bait on the hook. Her books are all about the psychological undercurrents, how the case breaks apart the detective narrator and makes it impossible for him to go on as he has been. The narrator of In the Woods, Rob Reilly (Killian Scott)—Rob Ryan in the books—has his own history, decades old, in the woods where Katy Devlin’s body is discovered on an ancient altar by a team of archaeologists. His partner, Cassie Maddox (Sarah Greene), with whom Rob has an enviably close and harmonious platonic relationship, is the only person who knows about Rob’s past, but even Rob doesn’t really understand what happened to him in the summer of his 12th year.

In the Woods doesn’t have enough plot for a conventional high-end, eight-episode thriller. Phelps might have tried to fix this by finding an unconventional way to tell the story, but instead she has grafted the plot of The Likeness, French’s second novel, onto In the Woods, making the events of both novels contemporaneous. The Likeness is narrated by Cassie, who learns that a murder victim had been living under a false identity that Cassie herself created while working undercover and then later abandoned. She agrees to pose as the victim, who resembles Cassie exactly, to find the killer. The premise is outrageously unbelievable, but in the dreamlike thrall of Cassie’s telling, the novel works.

Combining these plots is a terrible idea for multiple reasons. One is simply logistical; the fusion turns two improbable but engaging stories into a ludicrous farrago. No homicide detective would take a hiatus from an active investigation as sensitive as Katy Devlin’s murder to assume an undercover identity, not even for a weekend. More importantly, In the Woods is Rob’s story; The Likeness is Cassie’s. Each narrative has a dreamlike quality that only prevails if it’s allowed to unfold, unbroken in the voice of the character experiencing it. Switching back and forth breaks the spell and suggests a connection between the two stories that does not exist, the sort of baroque conspiracy that’s antithetical to French’s more intimate concerns.

French’s witchy explorations of subjectivity and its discontents could well be unfilmable. Seeing is believing, and uncertainty, the opposite of believing, is French’s game. But this attempt, despite strong performances from an excellent cast, doesn’t even come close enough to do her novels justice.