Criminologists know, but the rest of us perhaps forget, that America doesn’t have the serial killer market completely cornered.

Although we cram our prime-time TV schedule with fictional versions of them, any culture can harbor one. What’s interesting are the different ways societies dramatize and mythologize their murderers when the time comes to repackage the court transcripts and tabloid tallies of the grisly details for films and TV movies.

If anything, the methodically compulsive “Appropriate Adult,” airing Saturday night on Sundance Channel, is a study in artistic and narrative restraint: Although it is about one of Britain’s most notorious modern-day killers — Fred West, whose story held the U.K. in a thrall during the mid-1990s — it doesn’t feature a single murder. Corpses are dug up but never seen.

“Appropriate Adult” is also a chance to ponder some peculiarities of British criminal and legal procedure. Faithfully based on the story of West’s protracted confessions to torturing, raping, killing and burying 11 women between 1967 and 1987 (and, some still speculate, as many as 20 more never-found victims), “Appropriate Adult” has a documentary air about it and a plodding seriousness. West committed suicide in prison before he was tried; his wife, Rosemary, was convicted for helping torture and kill 10 of the victims — one of them the Wests’ 17-year-old daughter. The movie seems frightened of the Wests, which subtly translates to the viewer’s experience, as well. What’s most scary is the couple’s utter banality.

The continually evolving Dominic West (Detective Jimmy McNulty from HBO’s “The Wire”; star of BBC America’s “The Hour”) plays Fred West here — with the actor de-handsomed in the manner of Charlize Theron’s serial killer in “Monster,” aided by bad teeth and a Greg Brady perm. Sometimes, West gives into the temptation of letting his creepy makeover do the acting for him; at other moments, he is chillingly watchable.

Emily Watson as Janet Leach and Dominic West as Fred West. (Matt Squire/ITV/Sundance)

It is February 1994 and Fred, who had an unsavory criminal history, is arrested for the disappearance and presumed murder of his daughter years earlier. He confesses that his daughter is buried in the narrow yard behind the West residence — a home where prostitution, home pornography and incestuous rape were everyday events.

Emily Watson, purposefully hiding her luminosity, plays Janet Leach, a wife, mother and part-time student who has signed up to be an “appropriate adult,” which is a court-appointed volunteer citizen who sits in on interviews with suspects to safeguard against police intimidation or other legal rights violations.

Timid Janet gets assigned Fred West as her first case. From her first moments in his presence at the police station, she is at once horrified and fascinated. “Appropriate Adult” thus lands on its (apparently still controversial) narrative thread: Fred, desperate to protect his wife from arrest or conviction, draws on Janet’s sympathies, telling her things in confidence that she can’t legally tell the investigators.

On it goes, as Fred’s stories continue to spill out and the body count rises. “Appropriate Adult” is appropriately obsessed with Janet’s inappropriate fixation with Fred and his crimes. In a way, she becomes his final victim as the film harnesses the case’s troubling questions — how much was Fred lying? What really happened in the West household? Janet becomes consumed, even as her family suffers from her obsession. Like all real-life retellings, “Appropriate Adult” spreads it on a tad thick and never quite builds to a finish that would justify its 150-minute length. It gets the ambiguity right, but all that leaves us with is a lot of ambiguity.

Appropriate Adult

(21 / 2 hours) airs Saturday at 10 p.m.

on Sundance Channel.