When Olivia Cooke’s Amanda enters a lavish Connecticut home at the beginning of Thoroughbreds, it’s immediately clear something is off. It’s not just the eerie pops and crackles of the film’s score, or the fact that the opening scene showed her taking a knife out of her bag after menacingly staring at a horse. It’s that her face is entirely drained of emotion. When scanning the happy family photos of her childhood best friend’s home, Amanda suddenly looks up, cocks her head, and shoots a freakishly phony smile into a mirror. She holds it for an uncomfortable beat, then, as if deciding that particular emotion didn’t fit, she reverts back to her blank expression. Amanda is a sociopathic teen who tries on different emotions like clothes. But little does she know, she’s not the only one with sinister tendencies.

Playwright Cory Finley makes his directorial and screenwriting debut with Thoroughbreds, a deliciously twisted thriller and dark comedy about two wealthy suburban girls who plot a murder. At first Amanda appears to be the bad seed to her friend-turned-SAT-tutor Lily (Anya Taylor-Joy). We know something harrowing went down in that stable with Amanda and the horse by the way others act in her presence. Lily seems to be her complete opposite, a picture-perfect Westchester girl who attends boarding school (Amanda got kicked out), wears preppy clothes, and carries herself like a life-sized porcelain doll. But as the film proceeds through its four chapters, the baleful layers beneath Lily’s angelic facade shine through.

As Amanda and Lily spend more time together, their relationship evolves into a game of manipulation. In one scene they test who can hold their breath longer underwater until Lily nearly drowns; in another Amanda offhandedly suggests Lily should kill her stepdad Mark (Paul Sparks), an arrogant, short-tempered prick she and her mom can’t stand. Uncomfortable (but not nearly shocked enough by the suggestion) Lily kicks Amanda out of her house. But it’s a proposal she becomes fixated on after Mark tells Lily he’s shipping her off to an all-girls reform school.

Though dark and unnerving, Thoroughbreds is also viciously funny, thanks largely to Cooke’s deadpan demeanor, like when she nonchalantly lists her many mental health diagnoses (BPD, SAD, etc.), or begins to cry, only to snap out of it and reveal she’s trained herself to cry on cue.Additional icy comedy comes from Anton Yelchin, who gives a great performance in one of his final roles as a local drug dealer the girls recruit for their murder plot. It’s not a big part, but it’s a reminder of the wit the late actor often brought to his roles.

Taylor-Joy and Cooke bring a charged energy to their scenes, that’s well-matched by the unhinged score, which adds a thick layer of prickly anxiety to the film. But ultimately it’s Finley’s sleek and stylish visual language that makes Thoroughbreds a must-see, and one of the best surprises out of Sundance. He composes his shots with such precision, control, and confidence. He often places Taylor-Joy and Cooke on opposite edges of the frame, emphasizing their rivalry while injecting tension into the empty space between them. He uses minimal cuts and little camera movement; instead he maintains a steady focus on his characters through tracking shots and long takes that study his actor’s faces and build an air of suspense and unease. It’s stunning work, particularly from a first-time filmmaker. I cant wait to see what Finley does next. But in the meantime, we’ve found the first best film of the year.