In “Horse Girl,” Sarah (Alison Brie) is a mousy young woman undergoing a paranoid breakdown. As she spirals further from reason, the movie follows suit. We plunge into a prolonged nightmare of instability, both in terms of Sarah’s hallucinations and the movie’s reckless evocation of them.

Streaming on Netflix, “Horse Girl” opens in a lucid reality, if a quirky one. It’s Sarah’s birthday, and her plans are nonexistent. Shy and a little socially off, she fails to connect with her Zumba classmates, likewise her cool-girl roommate. She wards off loneliness through peculiar obsessions, particularly a fantasy TV series called “Purgatory” and the horse she once owned, which she visits frequently enough to annoy its new owner and stablemen. The director Jeff Baena, who co-wrote the script with Brie, gently lingers on Sarah’s hobbies, helping us understand how they serve as armor against an emotionally taxing world.