In the middle of winter, an American student named Dani is traumatized after her sister kills their parents and herself by Carbon Monoxide poisoning. The death of her family causes further tension to Dani's already strained relationship with her boyfriend Christian.
A few months later, Christian alongside his friends Josh and Mark are invited to join their friend Pelle in attending a midsummer festival taking place over nine days hosted by the Hårga, Pelle's ancestral commune, in Sweden. Josh is currently writing his thesis on midsummer festivals and regards the trip as a "once-in-a-lifetime opportunity" as the festival only occurs once every 90 years. Christian was planning on breaking up with a still grieving Dani before the trip but reluctantly invites her after they get into a fight.
Upon arriving to the commune, the group is introduced to Simon and Connie, a couple invited by Pelle's commune-brother Ingemar who offers the group psychedellic mushrooms. Dani has a bad trip and hallucinates her dead family.
The next day, the ättestupa ceremony occurs. This is a senicidic ritual in which community elders commit suicide by jumping off a cliff into the rocks below once they turn 72 and is considered a great honor. Two of the elders participated in the ceremony, but when one elder survives, the commune mimics his screams of pain before smashing his head with a mallet.
Distressed from witnessing this event, Dani is convinced to stay by Pelle. Simon and Connie decide they want to leave and are driven to a train station. Christian decides that he also wants to write his thesis on midsummer festivals and finds through his research that outsiders can be brought in for mating purposes to avoid incest. Mark unknowingly urinates on a sacred tree and is lured away by a commune member later that night. Later, Josh sneaks out of bed to take pictures of sacred texts and is caught and beaten to death by a half naked man wearing Mark's skinned face.
The following day, Dani and Christian are pressured to drink hallucinogenic tea. Dani is crowned May Queen after winning the maypole competition. Christian drinks the tea and is lured into a sex ritual.
Dani witnesses the ritual and has a panic attack, fleeing the scene and joined by some of the Hårgan women who mimic her cries. Christian tries to run away but instead stumbles upon Simon strung up in a barn as a blood eagle. A distraught Christian is caught by the Hårga.
In the final ritual, the Hårgans explain that they must perform nine human sacrifices to purge the commune of evil. So far, they have sacrificed eight between the two elders from the day before, two volunteers, and four of the guests brought by Pelle and Ingemar. Being crowned the May Queen, it is upon Dani to choose whether the final sacrifice will be Christian or a community member.
A paralyzed Christian is stuffed into a bear and placed inside a yellow wooden temple along with the other sacrifices. As the temple burns down, Dani sobs but smiles as the structure collapses, ending the film.