A devoutly religious Scottish police sergeant, dedicated to his beliefs and duty, receives an anonymous letter from a resident on the isolated island of Summerisle. The letter reports the disappearance of a young girl, Rowan Morrison, and pleas for help in locating her. Determined to solve the mystery of the girl's whereabouts, the sergeant travels to the island to investigate. Upon arrival, he is struck by the stark contrast between his own puritanical convictions and the islanders' pagan practices, which are centered around the cult of fertility and the worship of the natural world.

The sergeant's inquiries are met with evasion and outright lies, as the island’s inhabitants, including its leader Lord Summerisle, deny the girl's existence or claim she never went missing. He witnesses the island's strange rituals and customs with growing horror and disbelief, especially as they prepare for their May Day celebrations, which seem ominously significant. Convinced that Rowan's life is at stake, the sergeant's investigation becomes a race against time as he tries to unravel the island's secrets while struggling to maintain his own moral compass amidst the seductive and bewildering pagan rites.