A boating accident runs a young man and woman ashore in a decrepit Spanish fishing town which they discover is in the grips of an ancient sea god and its monstrous half human offspring.

Based on a short story by H.P. Lovecraft, the undisputed master of the macabre, Dagon tells the story of Paul Marsh, a young man who discovers that the truth will not set him free instead it condemns him to a waking nightmare of unrelenting horror. A boating accident off the coast of Spain sends Paul and his girlfriend Barbara to the decrepit fishing village of Imboca looking for help. As night falls, people start to disappear and things not quite human start to appear. Paul finds himself pursued by the entire town. Running for his life, he uncovers Imboca's dark secret: that they pray to Dagon, a monstrous god of the sea. And Dagon's unholy offspring are freakish half-human creatures on the loose in Imboca...

Fifteen year-old Emma Evans has an argument with her mother Lucy since she wants to go to a concert in London with her friends Rose and Alex. Emma immediately has a convulsion and her family takes her to the hospital, but the doctors do not find any physical problem. Then her father John tries to convince his wife to send Emma to school, instead of homeschooling, but she prefers to keep Emma in the family-based education. Lucy sends her daughter to the psychoanalyst believing that she has psychological problems and Emma asks her friend Rose to record her session of hypnosis though the cellular but the doctor dies during the session. When Emma listens to the tape, she believes she is possessed by the devil and asks her parents to be submitted to an exorcism with her uncle, Priest Christopher Taylor. However her skeptical mother is against the ritual and recalls that Chris was responsible for the death of a teenager, Ana, in the past in an unsuccessful exorcism. But when Emma levitates ...