Every person owns a phobia that chokes his or her mind. But there is one fear we all share: the unknown. Our species studies things to understand and control them. If we can’t, we become hopeless, confused and, of course, frightened. The unknown can do whatever it will to us. We can’t anticipate it coming until it’s too late.

The cold, eerie British film “A Dark Song” understands this principle perfectly. Liam Gavin’s confident first feature establishes tension and unnerving control over the audience from the start. Its descent into shock and impending doom is the sort of experience that will follow you home from the theater and nest in your nightmares. Because it moves beyond the boundaries of popcorn fright films, it also sends you away with an unexpected sense of grace. Labeling it exclusively as a horror film is too limited.

The story is grounded in our tangible real world among inconspicuous everyday characters. The opening scenes show the protagonist on a journey across bleakly beautiful north Wales in search of something unclear. Long shots of gloomy landscapes move us to search for something we hope isn’t there. Seeing the troubled expression of the woman driving across those somber backdrops emotionally invests us in the plight of Sophia (played with convincing gravitas by Catherine Walker), whose son died three years earlier. Inconsolable grief has emptied her soul. She is preoccupied with the afterlife, with death and heaven and hell. She is seeking something beyond Christian theology, a way to reach across the universe and contact her dead child.

To guide her along that path, Sophia gives her considerable life savings to Joseph (Steve Oram in a remarkable performance), a cryptic explorer of paranormal secrets. Initially, the scowling Joseph refuses the offer almost contemptuously. Sophia is proposing to enter a chaotic realm of demons and guardian angels and mysterious supernatural agencies. He will map out the passage and accompany her only if he feels she is worthy of the risk. With no other options, she resentfully complies.

Persuading and misleading each other about their motives, the two settle into the spooky, spiritually correct rural mansion house that Sophia rented. As they go through weeks of preparation, fasting and study, dread and predestined evil begin rising like an ancient force. Joseph keeps Sophia on a toxic diet of toadstools and blood, running her through mental mazes that shred the very essence her identity. The pairing feels less like master and pupil than master and slave, pushing both characters and the audience into excruciating paranoia.

The impossible happens after Gavin has taken us over so many hurdles that it doesn’t seem impossible anymore. The breathtaking finale carries us from the idea of establishing contact with the world beyond to an intriguing tale of good vs. evil.

“A Dark Song” is unusual for horror. Throughout most of its nerve-shredding length there is no violence, and it explains absolutely nothing. Gavin creates psychological terror that exploits our anxieties with symbolism, nuance and innuendo. That purposeful ambiguity involves the viewer more intimately and increases the power of the story.

While the film feels too long and too slow in parts, I couldn’t think of any episodes I would cut to speed it up. Many who watch it will likely enjoy it for different reasons and perhaps at different levels. Some will not tune into the wavelength of a horror without jump scares, without a profusion of gore (though when there is blood, it’s grisly), and without boundaries. This film didn’t just overwhelm me, it knocked me completely off my rocker.