When you boil a thriller down to the bare essentials, you realize just how many of them get by on one killer gimmick. Jimmy Stewart, busted leg, trapped in his apartment watching a murder and cover-up through his “Rear Window” or Glenn Close falling for her possibly murderous client in “Jagged Edge,” any hook that distinguishes your film from the pack is a good thing.

The Spanish thriller “The Chalk Line” lays out a pretty good hook right there in the title. A lost, disturbed child who refuses to speak was somehow conditioned to live her life within the confines of chalk lines that circumscribe her world. What monster abused her to live life this? Or what manner of monster needs those boundaries to protect others from her?

The debut feature of Spanish director and co-writer Ignacio Tatay doesn’t limit itself to those two possibilities, which is both an asset and a hindrance in this moody but frustrating mystery. And the more answers it provides in the third act, the more frustrating it becomes. Simply put, it doesn’t play “fair” by thriller rules, and fizzes out in a formulaic denouement.

Paula and Simón are driving to their remote Spanish home when they see a child wandering in the middle of the road at night. She survives the accident she causes, but neither the police nor the medical authorities charged with treating “Clara” (Eva Tennear) can get a word out of her.

And there’s this thing she does with chalk…

The couple (Elena Anaya, Pablo Molinero) takes an interest in what happens to her, the hunt for her family and her well-being. The child screams, lashes out and has to be restrained in the hospital. But the fact that she responds just a bit to Paula means they’ll take her home and attempt some sort of extreme “fostering” to keep her calm until she gets better and/or they discover who she belongs to.

“Simón and I will make sure nothing bad happens to you,” Paula assures the child, who may not even speak Spanish (you can also watch this dubbed into English). But first we, and then husband Simón, wonder if the source of anything “bad” might be Clara herself.

The script teases that something supernatural could be afoot, something innately sinister in the blonde girl whose first words seem random noises, unless you speak German.

And then the film abruptly turns in an entirely more conventional and less satisfying direction and the movie lost me.

Still, this idea of a scared or scary child who can be limited in her access to freedom, weapons or threats — either received or delivered — by chalk lines she draws, erases and redraws, is an intriguing twist that held my attention through the first act and again in the finale.

Anaya has to carry the movie, and she struggles to keep her all-trusting character interesting as Paula does her own digging into where this child came from and what made her this way. Bigger emotional moments were called for, better manipulations of suspense in the script and choosing shots and edits that heighten this alarming moment or that revelation would have helped.

“The Chalk Line” is never a complete write-off, but there is no getting around that it isn’t what it could have been.

Rating: TV-14, violence

Cast: Elena Anaya, Paulo Molinaro and Eva Tennear

Credits: Directed by Ignacio Tatay, scripted by Isabel Peña and Ignacio Tatay. A Netflix release.

Running time: 1:47