The film begins with a haunting mural depicting a surreal ritual, accompanied by the eerie sound of folk songs and imagery of dark, snow-covered forests. Dani Ardor (Florence Pugh) attempts to reach her family but is met with only voicemail. Concerned for her bipolar sister Terri (Klauda Csanyi), who recently left a troubling message, she also contacts her boyfriend Christian Hughes (Jack Reynor). Christian, who is out with his friends Josh (William Jackson Harper), Mark (Will Poulter), and Pelle (Vilhelm Blomgren), reassures Dani that her sister's behavior is just another episode, and advises her that she feeds into Terri's issues. Shortly after their conversation, troubling news arrives; authorities discover that Terri has tragically killed their parents in a carbon monoxide accident.
Months later, still grappling with her grief, Dani tries to find a sense of normalcy as she interacts with Christian and his friends. They are planning a trip to Sweden for a rare midsommar festival in Harga, a village rich in tradition, which occurs only once every 90 years. Initially invited out of sympathy, Dani catches Christian off guard as she expresses her desire to join, revealing her hurt over being excluded from such an important aspect of his life.
In Sweden, Dani meets new acquaintances, including Simon (Archie Madekwe) and Connie (Ellora Torchia). The festivities begin with the group indulging in psychedelics, which triggers a series of intense, perplexing experiences for Dani. Struggling with paranoia and hallucinations, she finds herself overwhelmed and disturbed by what she perceives as her deceased sister Terri. After vivid dreams and visions of her family's loss, Dani awakens next to Christian.
As the festival unfolds, the group witnesses bizarre cultural practices, leading to a shocking scene where two elderly villagers commit ritualistic suicide, plunging themselves off a cliff to symbolize the completion of their life cycles. While her friends are horrified by the event, Dani’s reaction is complicated, leading to more questions about the tenuous connection she shares with Christian. Meanwhile, tensions rise among the group, revealing a deep-seated discontent towards Christian for his inability to stand on his own two feet.
As the days pass, Dani participates in a folk dance around a maypole, where she becomes the last woman standing and is crowned the May Queen. However, the celebratory atmosphere takes a dark turn when she witnesses Christian engaging in an intimate act with Maja (Isabelle Grill), another villager. This betrayal pushes Dani further into despair.
Triumphing over personal loss, the festival culminates in a grotesque ritual where Dani is faced with making a harrowing choice involving sacrifices. Christian, along with other outsiders, becomes a target for the village’s sinister customs. In a chilling climax, he meets a gruesome fate, encapsulated inside a bear's corpse as part of the ritual. Stricken with horror yet slowly succumbing to the village's manipulations, Christian ultimately faces his own tragic end, while Dani, immersed in the villagers' unified wails, begins to allow a disturbing smile to surface, signifying a newfound acceptance of her harrowing reality.
