In the United States, college student Dani Ardor is filled with anxiety after receiving a disturbing email from her bipolar sister Terri. Dani reaches out to her boyfriend Christian, who dismisses her worries as overreacting. Christian secretly plans to break up with Dani and is packing for a summer trip to Sweden with his friends Mark, Josh, and their Swedish friend Pelle. That night, Dani's worst fears come true. Police discover that Terri committed murder-suicide, taping exhaust hoses to her mouth and pumping carbon monoxide into the house, killing both herself and their parents. Months later, Dani is paralyzed by grief. After finding out about Christian's secret trip to Sweden, Christian reluctantly invites Dani along out of pity and guilt. The group arrives in Hälsingland, Sweden, where Pelle's ancestral community, the Hårga, is celebrating a special midsummer festival that occurs once every 90 years. They are joined by a British couple, Simon and Connie, who were invited by Pelle's brother Ingemar. The visitors are given psychedelic mushrooms and welcomed into the commune. The sun never sets, distorting their sense of time. On the first full day of ceremonies, the community gathers beneath a towering cliff for the Ättestupa ritual. Two village elders who have reached the age of 72 climb the cliff and jump off to their deaths onto the rocks below. The elderly woman dies instantly. The elderly man survives the fall with mangled legs, screaming in pain until four villagers step forward with a massive wooden mallet and smash his skull to pieces. Simon and Connie scream in horror and demand to leave. The village elders calmly explain that completing life at 72 preserves the harmony of nature and prevents the indignity of aging. Simon is told he will be driven to the train station, but he disappears. When Connie later tries to leave, she also vanishes without a trace. Meanwhile, tensions flare between the American friends. Josh wants to photograph the Hårga's secret sacred text, the Rubi Radr, for his doctoral thesis. When the elders refuse him permission, Josh sneaks into the temple at night to take photos. A figure wearing Mark's skinned face attacks Josh, bludgeons him with a mallet, and drags his body away. Mark had already been murdered after accidentally urinating on the village's sacred ancestral tree. Christian is lured into the cult's web. A young village woman named Maja develops an obsession with him, placing love runes under his bed and baking menstrual blood into his food. The elders invite Christian to mate with Maja to bring fresh bloodlines into the community. Christian drinks an aphrodisiac potion and is led into a room where Maja lies naked, surrounded by a chorus of older women who breathe, push, and moan in unison with them. At the same time, Dani participates in the traditional Maypole dance competition. Consumed by the trance of the music, Dani dances for hours, outlasting every other woman and winning the title of May Queen. Dressed in an enormous dress made of hundreds of fresh flowers, Dani is celebrated by the entire village. While being paraded around, Dani peeks into the ritual house and catches Christian in the act of mating with Maja. Dani collapses outside in a violent panic attack. The women of the village surround Dani on the floor, locking eyes with her and screaming, sobbing, and hyperventilating in perfect harmony with her agony, validating her pain for the first time in her life. Drugged and disoriented, Christian runs outside naked. An elder blows a paralyzing powder into his face, causing Christian to collapse to the ground unable to move or speak.

The festival reaches its grand climax: the final ninth sacrifice. The elders reveal that nine human lives must be burned inside the yellow wooden temple to cleanse the village of darkness for another 90 years. Four lives come from outside visitors (Josh, Mark, Simon, and Connie, whose corpse was preserved in chainmail and Simon whose body was mutilated with the Viking blood eagle). Four lives come from the Hårga (the two cliff elders, plus two volunteers, Ingemar and Ulf). The ninth and final sacrifice must be chosen by the newly crowned May Queen between a local villager and an outsider. Dani is brought forward. Looking upon the paralyzed Christian, who abandoned her emotionally during her darkest tragedy, Dani chooses Christian as the final sacrifice. Christian is sewn into the hollowed-out carcass of a slaughtered brown bear and placed in the center of the wooden temple alongside the bodies of his friends. The two living volunteers are given paralyzing herbs to prevent pain, and the temple is set on fire. As the flames engulf the building, the villagers outside scream and mimic the dying screams of the volunteers inside. Dani watches the temple burn. She begins by crying in horror, but as the heat rises and the building collapses in flames, her weeping slowly transforms. A radiant, serene smile spreads across Dani's face. She has finally destroyed the toxic relationship that held her captive, purged her family trauma in the fire, and found a community that shares her grief and celebrates her as family.