Anya Taylor-Joy stars as Lily and Olivia Cooke as Amanda in THOROUGHBREDS, a Focus Features release. Credit: Claire Folger / Focus Features

If you weren’t already a little afraid of teenage girls, the new film, “Thoroughbreds,” might just change that.

Anya Taylor-Joy (“Split”) and Olivia Cooke (“Bates Motel”) are prep school seniors Lily and Amanda. The two were childhood friends before parental life decisions separated them. Circumstances bring the two girls back together, although the closeness they once enjoyed is different now.

Lily lives with her mother and step-father in a grand and sterile mansion in a Connecticut suburb. Her mother married into wealth and her step-father, Mark (Paul Sparks, “The Greatest Showman”), parades around in his bicycle shorts, drinking green smoothies and never seeming to actually go to work to pay for his comfortable lifestyle.

Amanda, on the other hand, lives with her single mother in more modest accommodations. But Amanda doesn’t care so much about that. In fact, Amanda doesn’t care much about anything. It isn’t a borderline personality disorder or depression or anything of the sort. She’s been tested. She simply lacks the capacity for emotion. A fact that fuels the rumors after she is arrested for animal cruelty when she euthanizes her own horse in a particularly brutal way.

And then, one day, as Mark hovers in the background, making his oppressive presence known, Amanda asks Lily the all-important question: “You ever think about just killing him?”

That question launches the story in a new direction, and leads the girls down a path that has been compared to some combination of “Heathers” and “American Psycho.”

“Thoroughbreds” is a drama, a thriller, and a dark comedy blended in one. Which is where comparisons to those aforementioned films are apt. But this is different. It is something else entirely. The two girls aren’t connected by a desire to make a name for themselves. One doesn’t try to lure the other into something to satisfy sociopathic boredom. Instead, Amanda, despite her lack of emotions, does still display some sense of wanting to help. There must be empathy somewhere buried in her psyche, and when it peeks out, it’s surprisingly unsurprising.

A number of films present tales of good kids taking dark turns. What sets this apart is the fact that Lily’s fantasies of murdering her step-father are fueled simply by the fact that he is just…off. It is a refreshing take. He’s not abusive in any physical or sexual way, as most evil movie step-dads are. The problem is simply that he is there. Even when he’s not. Even during the times he does go to the office, or take his bike out, or leave for the weekend. He is always there and always makes things uncomfortable. Sparks does a great job with this. He understands the balance between menacing and irritating, and he maintains that balance well.

Anya Taylor-Joy and Olivia Cooke are very different types of actresses, and here they get to showcase their unique abilities. While Cooke’s Amanda feels nothing, Taylor-Joy’s Lily seems to feel everything, and too intensely. Both are quite convincing, and the scenes they share are fun without ever straying into silly.

Director Cory Finley marks his feature film debut here. He wrote and directed “Thoroughbreds” with a surprising level of comprehension. Far too many stories about teenagers—particularly about teenage girls—focus on their apparently insatiable need to be validated by a romantic relationship. But Finley understands that girls care about other things besides love and lust. And here he allows the story to simply be about two girls who are friends and nothing more. Neither of them is dating anyone or trying to. Neither of them needs that. They simply exist together and help each other through various life experiences. It’s a little strange that a film about potentially homicidal teens can portray a refreshing reality about them, but there it is.

It is also important to note that this was the final film from Anton Yelchin (“Star Trek”), whose appearance onscreen is a bit jarring. He shows up as the local drug dealer-turned-gun-for-hire, and his scenes are good. But he is capable of more than what he is given, and we know it. This could almost have served as an interesting cameo had he lived.

The film isn’t perfect. Some scenes drag too long and the story feels like it loses its place on more than one occasion and has to remember what it was saying. But it is fun to watch. Anya Taylor-Joy and Olivia Cooke prove they are going to stick around awhile. And, hopefully, this will be only the first of many interesting films from Cory Finley.

“Thoroughbreds” is distributed by Focus Features and will be in theaters Friday.

GRADE: (★★★)