“I found myself here,” says the bewildered boss turning up unannounced at his attractive secretary’s flat one evening. If even he can’t fathom the workings of his own libido then god help his patients, for David Ferguson is supposedly a high-flying psychiatrist. “We’re just as messed up as anyone else, just better at hiding it,” he explains. Except he isn’t.

Adapted from the novel by Sarah Pinborough, the series kicks off with what feels like a familiar emotional triangle: a man and woman have an affair, in the course of which the woman becomes fascinated with the cheated wife. Louise, the secretary, is the most sympathetic character as she tangles with the attractive couple who’ve just moved into the area. Both are slippery individuals, hard to read: Adele (Eve Hewson) has a history of mental collapse, while David (Tom Bateman) is the doctor/lover whose ministrations have more than a hint of Gaslight about them. In private, husband and wife say things like “We can never tell anyone what we’ve done!” David’s close-up sex-face is frankly terrifying.

Adding to the unease is the persistent tension of social inequality. Louise (Simona Brown) has a nice flat in what looks like a former council block, while David and Adele live in a palatial Islington villa. Though David must earn a bob or two as a shrink, it’s mostly Adele’s money; far from checking her own privilege, she refers to her inheritance as though it’s an adorable idiosyncrasy, like a cute lisp. On the lowest rung of society is Rob, the former heroin addict who fell for Adele in rehab. Rob has mysteriously disappeared, and may have been the couple’s victim.

Despite Adele’s eerie intensity, the women bond over their shared night terrors. For the first few episodes, unless you like sex scenes featuring David’s scary visage, the only real excitement comes from dream sequences, in which Louise repeatedly runs down a burning corridor to cries of “Mummee!” from her seven-year-old (Tyler Howitt, a stand-out). Otherwise it’s all leisurely coffee sessions, strained office interactions with the hopelessly unprofessional David, and maternal hugs at the school gate.

Stray details that turn out to be clues are only elucidated by the shock ending. It is indeed sensational, though the sensation is just the stunned feeling you get when you smack your forehead repeatedly on the coffee-table in furious disbelief. It is probably intended to make you want to go back and watch the whole series again with fresh eyes (remember the title?). The only problem is, the run up is so ponderous you might not even make it as far as the twist in the first place.

★★★☆☆

On Netflix from February 17