UNEXPECTED TWIST: Yellowstone star Kelly Reilly once traded the ranch for pure terror — leading one of the darkest horror films ever made.

Before she became the fierce Beth Dutton, Reilly delivered a chilling performance that still leaves audiences shaken to this day.

Kelly Reilly will no doubt be familiar to devotees of the Taylor Sheridan television empire, as she played the morally complex Beth Dutton for all five seasons of the neo-western series “Yellowstone.” The ruthless ranch owner became such a popular character among the show’s roster that she even has a spin-off series of her own in the works, alongside Cole Hauser as Beth’s husband, Rip Wheeler. Outside of her television work, Reilly’s filmography includes collaborations with filmmakers like Joe Wright (“Pride and Prejudice”), Ciáran Foy (“Eli”), Kenneth Branagh (“A Haunting in Venice”), and Guy Ritchie (“Sherlock Holmes”), among others. But it’s her pairing with director James Watkins that not only serves as one of her best performances, it also happens to be one of the most uncomfortable horror movies of the 2000s. Let’s talk about “Eden Lake.”

The 2008 horror-thriller sees Jenny (Kelly Reilly) and Steve (Michael Fassbender) making a drive to the English countryside for a weekend getaway. Beyond the hustle and bustle of city life, the remoteness of Eden Lake serves as the perfect place for Steve to propose. The happy couple’s peace and quiet seems to be going great, until a group of local teenagers set up camp on the other side of the beach. Their loud music, coupled with an aggressive unleashed dog, breaks the romantic spell, to say the least. It’s mostly the kind of nuisances you’d expect from rowdy teenagers with nothing to do. But things start to escalate in a manner none are prepared for.

The couple’s tent is disturbed in the middle of the night, their food is infested with insects, and their car is taken for a joyride. Steve confronts the kids about getting the car back, which leads to a scuffle that ends in the dog’s death. Brett (Jack O’Connell), the sociopathic de facto leader of this youthful collective, considers this the point of no return and wages war on the couple. What follows is a harrowing survival movie that sees both factions make a series of gruesome decisions that will affect the rest of their lives.

Kelly Reilly takes a nightmarish getaway in James Watkins’ Eden Lake

I remember being incredibly skeptical when it was announced that Watkins would be helming the remake of Christian Tafdrup’s Danish thriller “Speak No Evil.” It almost seemed like a fool’s errand to potentially water down one of the bleakest horror movies of the 2020s thus far, but after seeing “Eden Lake,” it became easy to see how he came to tackle the project. Watkins’ brutality smartly weaves class, peer pressure, and cycles of violence in a vicious survival thriller package that evokes the grimy sadism of “Straw Dogs” and “Deliverance.” It’s a much more interesting film than the “screw them kids” backwoods horror movie I was anticipating. “Eden Lake” received great reviews upon its initial release, yet there was an air of controversy surrounding it — not so much because of the prolonged torture sequences, but because of its statements about class.

“Eden Lake” was considered by some critics to be a nihilistic condemnation of the British working class that went too far. It’s an easy read to make considering the film, on its surface, is about a pretty well-off couple becoming the source of cruelty once they’ve entered the lower class-occupied countryside. But “Eden Lake” is much more nuanced than that. It’s a terrifying film about every party making the wrong decisions, which leads to even further cruelty. As audience members, we can see all of the ways in which this could have been avoided. The couple isn’t squeaky clean here, as Steve not only digs his heels in, he also takes to breaking and entering into one of their houses as some kind of reclamation of his masculinity.

Reilly is great in this because, unlike Fassbender’s Steve, she plays a schoolteacher who understands the complexity of emotional imbalance in adolescents. She’s the most rational character in the film, so it’s devastating to see her witness such increasingly disturbing acts that shatter her world. Even the children aren’t all presented as sadistic, bloodthirsty monsters. They know they’re in too deep at a certain point, and you see their consciences working overtime as they continue to lash out against their detractors.

Within all of the genre checkpoints of “Eden Lake” is another horrific underpinning about the nature of accountability. The film’s opening sees Jenny and Steve stop by an outdoor restaurant, where some kids are running around. Steve’s instinct is to tell his potential spouse that they ought to be slapped, only to have the parent’s smack across their child’s face kill the sentence in its tracks. It mirrors the cruel finale in an interesting way that I dare not spoil. Let’s just say Watkins’ nihilistic series of events leaves you feeling worse than you came in. And it hurts so much more because of Reilly giving such a vulnerable performance that’s hard to shake.

“Eden Lake” is currently streaming on Prime Video, Hoopla, and Kanopy.