“Thoroughbreds” marks an auspicious debut for writer/director Cory Finley.

Coming from the world of theater, writer/director Cory Finley has made a masterful first film. His dark comedy/thriller is utterly enthralling, as much for what it doesn’t show as what it does. The story, concerning two teen girls (Olivia Cooke and Anya Taylor-Joy) plotting to kill a ruthless stepdad, could have been another potboiler, like so many thrillers that have tread similar terrain. Instead, Finley creates a calm, contemplative film, full of tension and thrills done in disquieting, economical ways. More is said with looks than words, and all the violence happens off screen. It’s a sly work, both theatrical and sensitive in its portrayal of privilege and murder.