Plot

A dinner party in a country house that sees four friends come together for a birthday celebration. But as the night progresses secrets emerge and unsettling events begin to unfold around them.

Set over the course of twenty-four hours, BARBARIANS sees couple Adam (Rheon) and Eva (Moreno) wake up in their supposed dream house on Adam's birthday. Lucas (Cullen), property developer and friend of the couple, arrives for dinner with his actress girlfriend Chloe (Spiridonov), to celebrate Adam's birthday and the couple's buying of the house. But secrets unravel over dinner, and when the doorbell rings the evening takes a nightmarish turn. As manners give way to madness, an 'idyllic' evening of celebration descends into a dark night of terror; and the group's civilized dinner party turns out to be anything but.—IFC Midnight

A video is shown advertising a property called the "Gateway" property, named after the nearby Gaeta stone - known as a sacred site to historical druids.

Going on a morning jog on his birthday, Adam, the current leaseholder of Gateway, comes across a wounded fox snared in barbed wire. He attempts to free the fox, but backs off when the animal hisses at him before resuming his jog. Returning home some time later while his girlfriend Eva works on a large replica Gaeta stone in the property courtyard, Adam discovers the dying fox in his kitchen. Eva and worker Dan come inside and, after expressing horror at the sight of the animal, Dan quickly euthanizes it.

Meanwhile, Lucas (the owner of Gateway and presenter of the advertisement) and girlfriend Chloe drive to the Gateway property to celebrate Adam's birthday. While on the way, they stop at the Gaeta stone as a neopagan celebration takes place. While there, Lucas is attacked by a celebrant while recording a video lamenting the death of Alan Wickes, his former partner.

Lucas and Chloe arrive at the property, and Eva takes Chloe to her art studio after learning of the latter's interest. Eva remarks that her largely dark and disturbing art has been focused towards pagan imagery since her move to Gateway. At the same time, Lucas gifts Adam a mysterious drug contained in an eye dropper. Later during dinner, Chloe admits to being pregnant. After she goes to the bathroom, Adam confronts her with worries that the child could be his. They return to dinner, and Adam spikes Lucas's drink with the drug in revenge for Lucas's taunting behavior. Lucas attempts to renegotiate the sale of the property, and a fight breaks out between him and Adam. Adam reveals his prior affair with Chloe as the doorbell rings. Eva opens the door to reveal two assailants in animal masks.

The invaders take Adam, Lucas, and Eva hostage while Chloe drives off, but is run off the road by another masked individual who takes her hostage with the rest of the group. The bird-masked invader, seemingly the leader of the group, performatively vandalizes the house before seating Lucas in a chair and dousing him in blood. Lucas is forced to read a confession letter in the style of his advertisement for Gateway, but shortly after begins to suffer from a bad trip and is able to break free from his restraints, killing the bird-masked invader. One invader chases Lucas outside while the other is overpowered and is revealed to be Dan. Lucas is overpowered and killed by his assailant Neil, who reveals that the invaders are the sons of Alan Wickes, and that they blame Lucas for their father's death and the loss of their family property.

Adam convinces the two invaders that he will sign the property to them. A conflict ensues in which Chloe stabs Dan who shoots her in retaliation, killing her. Eva and Neil fight before Adam tackles him out of a window, seemingly killing them both. Eva pursues Dan but is unable to bring herself to kill him. Adam is revealed to be alive, and decisively kills Dan with a captive bolt pistol before embracing with Eva. (thanks to Wikipedia)