Setting the story in a small picturesque village built into the Andalusian mountains of southern Spain, FERIA: THE DARKEST LIGHT, is a thriller that begins the morning after sisters Eva (Ana Tomeno) and Sofía (Carla Campra) spend the night at a bonfire party at the local quarry. They're frightened awake by the Spanish Civil Guard armed in tactical gear, raiding their home. Sitting down to speak with Guillén (Isak Férriz), the lead detective, they learn that their parents are the suspected leaders of a well-established demonic cult. A ritual sacrifice leaves the bodies of 23 members bleeding out near the entrance of a long-shuttered local mine just outside Sevilla. He shows them the security tape confirming their parents' involvement, leaving them even more confused. The sisters remain home in Feria, and end up expressing their grief and confusion in very different ways. Leaning on sibling stereotypes, Eva, the eldest, tries to stay pragmatic. Sofía, though, tries to understand her parents' disappearance by sinking deeper into the mystery alongside other cult followers.