On April 29, Sergeant Neil Howie embarks on a journey via seaplane to the secluded and lush island of Summerisle in the Hebrides to look into the mysterious disappearance of a young girl, Rowan Morrison. This investigation is prompted by an anonymous letter that raises concerns. A pious Christian, Howie is unsettled to witness the residents honoring ancient pagan Celtic deities in a place where churches have become relics of the past. The island's inhabitants engage in open sexual practices in the fields, incorporate children in their May Day festivities, educate them about the phallic significance of the maypole, and bizarrely use toads in throat treatments. The islanders are evasive, asserting that Rowan is a figment of his imagination.
During his stay at the Green Man Inn, the landlord's daughter makes a bold pass at him, but Howie, determined to maintain his chastity until marriage, declines her advances. While inspecting a collection of photographs that commemorate the annual harvest, he is puzzled to find the latest image of the May Queen missing; the landlord casually explains it was broken. At the local school, his inquiries about Rowan yield denials from the children, yet his investigation leads him to discover her name in the school register. The schoolteacher eventually points him to her grave.
By April 30, Howie confronts the island's leader, Lord Summerisle, a descendant of a Victorian agronomist, seeking permission to exhume Rowan's grave. Summerisle reveals that his grandfather introduced fruit breeds suited for Scotland and cultivated a belief that reviving ancient gods would bring prosperity, which swayed the islanders away from Christianity. As a result, the population gradually adopted pagan practices, while ministers were driven back to the mainland.
Upon exhumation, Howie finds the coffin surprisingly empty except for a hare's remains. Additionally, he uncovers the missing harvest photograph showing Rowan surrounded by empty boxes, indicating a failed harvest. Through his findings, he realizes that a human sacrifice is performed to appease the gods whenever crops fail, leading him to deduce that Rowan is still alive and is destined for sacrifice to secure a bountiful harvest.
On May Day, desperate for help, Howie attempts to return to the mainland but discovers his seaplane inoperable and its radio damaged, rendering him stranded. During the festivities, Howie overpowers the innkeeper, disguises himself in a Punch costume, and merges into the May Day parade, eventually locating Rowan. After a tense moment where he frees her and they escape into a cave, they are intercepted by the islanders, who joyfully welcome Rowan back.
Lord Summerisle informs Howie that he is the true intended sacrifice, as he fulfills their gods' demands: arriving of his own volition, embodying the authority of a lawman, retaining his virginity, and being duped into their trap. Howie warns Summerisle that the failing crops indicate the villagers will turn against him and offer him as a sacrifice if the ensuing harvest does not prosper, but his warnings fall on deaf ears. In a tragic culmination, the villagers force Howie into a large wicker man alongside various animals, set it ablaze, and sing the ancient folk song "Sumer Is Icumen In." Trapped within, Howie recites Psalm 23, praying fervently as flames engulf him and the animals, ultimately claiming their lives as the wicker man’s structure succumbs to the fire, silhouetting the setting sun.
