In 1912 in Switzerland, Charles Castle, a photographer, and Anna-Marie, his fiancée, are married in a small Alpine church. The next day they wander the mountains as a snowstorm closes in, and on their way back to the village a crevasse opens beneath them. Anna-Marie slips and falls to her death, despite Charles’s desperate attempt to pull her free.
During the Great War, Castle serves as an army photographer in the trenches of France. While he films corpses with his assistant Roy, a mortar shell lands nearby. Roy retreats to the trenches while Castle remains unusually calm, continuing to photograph as the danger closes in, and he only moves back to the trenches moments before the shell detonates.
After the war, Castle and Roy run a London photography studio, where Castle specializes in photomontage and other trick techniques. He attends a lecture at the Theosophical Society, where Arthur Conan Doyle examines a projected image of the Cottingley Fairies. Conan Doyle is persuaded of their reality, but Castle publicly debunks the image and distributes his business cards to the audience.
Back at his studio, Beatrice Templeton visits and shows a photo of her daughter, convinced that a mysterious shape is a fairy. Castle initially writes off the idea, but when he studies the image again he notices the shape reflected in the girl’s eye and proceeds to produce a number of large prints to reveal how the picture was made. Unable to explain or debunk the photograph, Castle travels to a village called Birkenwell to Beatrice, where he encounters Templeton’s daughters, Ana and Clara, and follows them home. Beatrice explains that the photograph no longer matters—she has seen the fairies—and asks Castle to meet her at the great tree in Birkenwell Woods the following day.
At the appointed time, Castle walks toward the great tree, where Beatrice waits. Before he arrives, she removes her hat and shoes and climbs the tree. When he reaches her, he discovers Beatrice has removed her clothing and lies lifeless on the ground. After making a statement at the local police station, Castle meets the Templeton girls, who are welcomed by their father, Nicholas, a Christian minister.
Castle learns Beatrice had been tracking her daughters’ odd behavior and discovers in her notes that she had been experimenting with a distinctive rare flower. Ana and Clara begin to ingest the flower themselves, and Castle takes some as well. The flower allows him to see the fairies, confirming Beatrice and her daughters’ visions.
Castle arranges a final, high-tech photo shoot to capture the experience, bringing in his partner and assistant. After another dose of the flower, they photograph what they see and Castle concludes that the fairies exist and that the flower slows the brain enough to perceive them—creatures that typically move far too quickly for ordinary cameras.
His obsession deepens as Nicholas grows tired of the project and starts burning his equipment. The flower’s influence over Castle intensifies, and when some fairies drift too close and catch fire, he erupts in a rage and kills Nicholas, then is arrested.
He refuses to defend himself, is found guilty, and sentenced to hang, while Ana and Clara are placed in foster care, seemingly unmoved by the outcome. Castle faces his death without fear, exchanging farewells with those around him. The film then flashes back to the Alps, where Castle struggles to save Anna-Marie again; this time he succeeds in pulling her back onto the path, and they embrace before continuing their walk.
