In 1905, Thomas Richardson (Dan Stevens) goes to an isolated island in Wales to save his beloved sister, Jennifer, after being kidnapped by a mysterious cult claiming reward for her release. Thomas is a former missionary who was tortured when he tried to introduce the Christian religion into Peking (the current Beijing) during a great anti-colonial and anti-Christian uprising, and has since abandoned his religion and faith.
Thomas arrives on the island pretending to be a believer to their Prophet, Malcolm Howe (Michael Sheen), who said he was wrecked with two jailies, Frank and Quinn. Howe also claims that the goddess of the island speaks through him. People in worship also refuse to pay taxes to the King. The island was uninhabitable in the past due to the fact that the soil is infertile. However, Malcolm managed to make crops grow by sacrificing animals and using his own blood.
Now the crops have begun to destroy again. Faced with great famine, the Prophet Malcolm conceived a plan to kidnap Jennifer and ask for money in exchange to feed his people. Thomas then saw the blood sacrifices people make in the cult, and in another house he notices Malcolm climbing into the attic through a hidden door. Thomas then starts the information gathering mission to find out where his sister is being held and to try to save her from a terrible fate.