The cornerstone of a good thriller is right there in the title to “Locked In,” a Famke Janssen star vehicle that loses track of the star almost as quickly as it tosses aside that essential building block.

This British thriller from Rowan Joffe, the nepo baby screenwriter who gave us George Clooney’s “The American,” as well as “Brighton Rock” and “The Informer,” sets up as the story of a “has-been” actress and hit-and-run victim who suffers from “locked-in syndrome.” Katherine drifts in and out of consciousness, perhaps “cognitively intact,” perhaps “vegetative,” and either way with no powers of movement, speech or telling anyone what happened to her.

But there’s this Irish nurse, Nicky (Anna Friel) who is assigned to Katherine (Janssen). She assures Katherine’s former ward, Lina (Rose Williams) that “if she’s in there, I’ll find her.”

As the first scene, which we see through Katherine’s blinking “one good eye,” has her code-spelling out “M-U-R-D-E-R” for Nurse Nicky, we know a mystery’s to be solved, and suspect young Lina might not want that to happen.

There’s a pretty good movie in the idea of a curious, slow-to-catch-on nurse helping her trapped patient cry for help and justice in piecing together blinked “alphabet board” coded messages about who put her in this spot. There are plenty of places for suspense, surprise twists and with our disabled “heroine,” perilous threats which she’s helpless to fend off.

Joffe and director first-time feature director Nour Wazzi instead give us endless flashbacks, most of them of Lina, the “heroine” of her own version of events, relating the story of how she came to be in the care of widowed Katherine, how Lina married Katherine’s seizure-prone, inherited-all-Daddy’s-money stepson (Finn Cole), and the doctor (Alex Hassell) who came into this world and played a role in the intrigues and tragedies to come.

“Locked In” struck me as one blown opportunity after another. The nurse is instantly suspicious and so brazen in her voicing of those suspicions that the film loses any sense of mystery about where it is going. We never fear the nurse, nor do we fear for this apparent “truth seeker.”

Yes, there are twists. But taking this tack eliminates one potential suspect and reduces the nurse’s role in the plot. As there are only three other characters, that’s going to be an issue.

Hassell, Cole and Williams stand out in the cast. But not much.

The snobby, fox-hunting country setting and haves-vs-have-nots friction are introduced but underdeveloped as the picture stumbles through longer and longer flashbacks to cast suspicion and aspersions on everybody we meet.

And the climax is so anti-climactic that twists or no twists, it just leaves the viewer frustrated that we didn’t get a tale of rising suspense, rising stakes and increasing peril instead of the movie they chose to muddle out of this promising premise.

Rating: TV-MA, violence, sex

Cast: Rose Williams, Alex Hassell, Anna Friel, Finn Cole and Famke Janssen

Credits: Directed by Nour Wazzi, scripted by Rowan Joffe. A Netflix release.

Running time: 1:36