Beth Boothby [Brittania Nicol] is a successful born-again evangelical pop singer from Texas. She and her fiancé Steve Thomson [Henry Garrett] both wear purity rings and belong to a group known as the Cowboys in Christ, who travel to heathen areas of the world to preach Christianity. The Reverend Moriarty sends them off to Glasgow, hoping to save some souls there, but they are met with a very negative reception as nobody accepts their pamphlets. They are approached by Sir Lachlan Morrison [Graham McTavish], the laird of the small village of Tressock in the Scottish Lowlands, and his wife Delia Morrison [Jacqueline Leonard], who invite them back with them to preach. They actually intend them for a more central part in Tressock's May Day celebration.
The villagers of Tressock have become infertile after the construction of Sir Lachlan's nuclear power plant, a detail that hangs over every interaction with the locals. While out riding a horse, Steve Thomson [Henry Garrett] has sex with female villager Lolly [Honeysuckle Weeks], whom he finds bathing nude in a spring. Steve regrets his actions and wants to return home. During a flashback Sir Lachlan remembers a mentor from his youth, a memory that hints at the deeper beliefs driving the village’s modern ritual life. Meanwhile, a detective named Orlando is sent to Tressock, posing as the local police officer to secretly investigate reports of a pagan cult. After having sex with Lolly on multiple occasions, Orlando discovers that the people of the village worship the ancient Celtic goddess Sulis.
Beth and Steve decide to begin their preaching at Tressock's May Day celebrations. To impress the locals, they agree to become the local Queen of the May and the Laddie for the festival, not realizing the consequences of this decision. Steve is chased by villagers on horseback as part of a ritual and is torn apart by them at its completion. Back in Sir Lachlan's house, the Morrisons' butler Beame [Clive Russell] attempts to sedate Beth to prepare her for her role as the May Queen. He had tried this the night before, but the spiked milk killed the Morrison's cat. Beth attacks Beame and flees, but is captured in town. After discovering Steve's death, Beth confronts Sir Lachlan at the wicker tree. She [Beth Boothby] pushes Lachlan into the structure and sets it on fire, killing him.
Beth tries to escape from Tressock with the help of one of the few children left in the village. She is captured and later killed. Her body is preserved and put on display in a room with the previous May Queens. Lolly gives birth to Steve's child and brings a new generation to Tressock for the first time in years. Delia Morrison [Jacqueline Leonard] prays to the setting sun for the gods to find more men to bring to Tressock to sire more children.
