Dani Ardor was depressed after his younger sister killed herself while killing her parents. This incident worsened Dani’s relationship with his less sensitive lover, Christian Hughes, an anthropology graduate student. Christian had long wanted to break up, but maintained his relationship because he felt responsible for taking care of Dani.
The following summer, Dani learned that Christian and his friends, Mark and Josh, were invited by their Swedish friend, Pelle, to attend a midsommar celebration in the ancestral village of Pelle, Hårga, in Hälsingland. Pelle explained that this celebration is done once every 90 years. Christian doesn’t tell Dani about his trip and they bicker. As a form of apology, Christian was forced to invite Dani to come along. Dani was willing.
They flew to Sweden and arrived at Hårga. They meet Simon and Connie, a British couple invited by Pelle’s sister, Ingmar. Ingmar offers them psilosibin mushrooms. As a result of consuming the mushroom, Dani hallucinated seeing his sister. This group of migrants broke up after witnessing ättestupa, a ceremony that required two village elders to commit suicide by jumping off cliffs. When the male elder did not die, the villagers imitated the sigh of his suffering and hit his head with a large hammer. The village leader, Siv, explained that deaths like this were common in Hårga and every citizen would do the same thing when he was 72 years old. This ceremony made them waver, especially Dani, but they decided to stay because Pelle persuaded them and Josh raised this tradition as the topic of his thesis. Simon and Connie decide to leave. When Connie was cleaning up, an elder told him that Simon had left first to the train station. Confused Connie finally left alone.
Christian decided to raise Hårga as the topic of his thesis, but Josh strongly objected. Josh tried to find additional information about the ancient practice of this village. This practice is based on the painting of a citizen who was born from an incest relationship and is considered a shaman. After Mark accidentally urinates the sacred tree and makes the people angry, he is invited by a woman he had estimated. That night, Josh sneaks into the village temple to take photos of their holy books. He was approached by a half-naked man who wore Mark’s skin and feet. Josh’s head was hit with a hammer, then his body was dragged out of the temple.
The next day, Dani was asked to consume psychedelic drinks and take part in a dance competition around the pole. He won and was appointed as “Queen of the May”, an honorable title in this village. At the same time, Christian was sedated and forced to attend sex rituals to impregnate Maja. A number of women witnessed and interacted in this ritual. After seeing Christian and Maja having sex, Dani panics and several Hårga women roar with him. Not long after, half-conscious Christian found Josh’s buried leg and Simon’s body cut off according to the bloody bird ritual. Christian is then paralyzed by an elder.
The villagers gathered and explained to Dani that nine humans had to be sacrificed in order to eliminate the evil influence from this village. The first four victims were migrants – Josh, Mark, Connie, and Simon – invited by Pelle and Ingmar. The other four victims were villagers – two of the sacrificed elders, Ingmar, and one other villager. Dani, as Queen of May, may choose the ninth victim from the immigrants or residents. Dani sacrificed Christian to eliminate him from his life. Paralyzed, Christian’s body was put into a bear’s body that had been emptied, then placed in the temple along with the bodies of the other victims plus two residents who were still alive. An elder gave poison to the two residents so they would not suffer. The temple was then set on fire. When the fire started to burn one of the residents, he shouted. Outside, residents joined in shouting to imitate his roar as a sign of the end of this ritual. Dani also cried, then slowly smiled.