It’s quiet almost all the time in the sprawling Connecticut mansion Lily (Anya Taylor-Joy) shares with her widowed mother (Francie Swift) and new stepdad Mark (Paul Sparks) in Thoroughbreds, the directorial debut of playwright Cory Finley. But for Lily — who seems to be on a break from her upscale boarding school — it’s not always quiet enough. She likes to sulk around and watch old movies, but sometimes the thrum of Mark’s exercise machine cuts into the silence. And that reminds her of how she hates Mark, who’s pretty much an asshole to her and her mother all the time. And that reminds her about how she and her mother only get to stay in that big, quiet house because of Mark’s money, which he feels allows him to treat them like an asshole all the time. And that makes her think there might be some way to make the house quieter.
Eventually. Lily doesn’t get to that conclusion on her own. In fact, as the chilly, darkly comic Thoroughbreds opens, foul play seems to be the last thing on her mind. She seems meek and passive, especially compared to Amanda (Olivia Cooke), an old friend with whom she’s reluctantly reunited to serve as a tutor. Where Lily will eventually start contemplating murder, Amanda already has blood on her hands, even if it’s not human blood. Amanda too is taking a break from school, her break necessitated caused by her killing a horse, an act that’s made her a town scandal.
Still, with the added incentive of some money, Lily picks up their relationship and they start talking, with Amanda playing the aggressor, bringing up her notorious act to shock Lily, pushing Lily about how she really feels about her stepfather, and revealing that she’s spent her life simulating emotions she can’t feel, even demonstrating how easy it is to make yourself cry if you know the right tricks.
This might seem like posturing if Cooke wasn’t so convincing. More than once, we see her looking into a mirror, practicing how to smile, and never quite getting it right. She may not be a sociopath — later scenes reveal some undisclosed motives for her animal cruelty — but she comes close. Then again, maybe it’s not that hard to tip into and out of sociopathy, as Lily discovers the more time she spends with Amanda. And maybe, they discover, it’s not that hard to merge into a kind of murderous hive mind as they begin talking themselves into plotting Mark’s demise, then roping in Tim (Anton Yelchin, in a memorable posthumous performance), a low-level drug dealer and sex offender to make sure their own hands stay clean.