If you watched the trailer before deciding to watch Horse Girl, then you likely had at least some idea as to the horror turn the story would take. The beginning of Horse Girl is more than normal — Alison Brie plays Sarah, a craft store employee who'd rather watch true crime docs and hang out with her childhood horse, Willow — who is now owned by a new family who doesn't love Sarah's obsessive tendencies — than have any sort of social interaction (just like many millennials).
At work one day, Sarah's co-worker Joan (Molly Shannon) notices a horse as it passes by the store window. She only gets a glimpse of the tail end of the horse (pun intended).
Things begin to go sideways for Sarah when her roommate Nikki (Debby Ryan) sets Sarah up on a date with her boyfriend's friend. While hanging out with said suitor, Sarah gets an impromptu nosebleed, and the mood in the film completely shifts.
Sarah is also having recurring nightmares about being in between two people in a white room, and she starts waking up in places with no idea how she got there. Her body is covered in bruises and she doesn't know where they came from.
It's revealed that Sarah's mother killed herself a year earlier, and Sarah isn't sure if her mental health is slipping or if something more sinister is happening to her.
She theorizes that aliens are messing with her because of her strong resemblance to her deceased grandmother.
After even more consideration, Sarah concludes that she is actually her grandmother, and she's been time traveling because of the aliens who have harmed her. Not-so-shockingly, this theory lands her under psychiatric evaluation.
Somehow, things only get stranger in the mental institution. Sarah fantasizes about a lot of things while she's there, and it's unclear what the audience is supposed to believe. She believes time is on a loop, which explains why she has memories of certain events while others around her don't.
Sarah escapes the institution and then dresses up as her grandmother in an effort to trick the aliens.
She frees Willow and passes her craft store on the way. This was the moment Joan had seen in the beginning of the movie, which seems to confirm Sarah's belief that time is on a loop.
After taking Willow to the woods, Sarah lays down while she waits for the next alien abduction and the film concludes as she's taken up to space.
There's never confirmation whether or not Sarah made the entire ordeal up in her head. The two main theories about the ending are split depending on whether people believe the scenes involving supernatural elements were her delusions or that they actually did occur.
The moments when Sarah woke up and didn't know where she was might have been botched alien abductions, which interrupted the time loop (and thus explain why she knew certain things happened while others didn't).
