A mysterious murder. Cruz, a police officer, goes to find his lover's husband in te middle of the mountain. The guy is hiding, or trying to die, not sure. But he's acting weirdly and is a suspect. He ends up in a crazy bin. But the murders continue. There are suspicions of a monster, but it's not clear if it's supposed to be something in the mountain or if it is someone.
It's quite confusing because at any point, every character looks like they could be the monster or under the monster's control, or even behave like they think they could be the monster involuntarily. Because of that throughout the movie and specially towards the end none of the character's behaviour makes sense according to what they know or suspect.
And to be clear, it doesnt feel like a deliberate intricate plot to make the viewer wonder, no, it just feels random.

And then out of nowhere there IS a monster [spoiler]that is none of the characters[/spoiler]. It looks weird (well weird as a monster of course, but also weird like it's anatomy seems really impractical) but not cheap, at least it's well done. Its interaction with the main character is totally not in line with everything we've seen until now so this adds more randomness to the story.
It finishes on useless long close up on the monster, which looks more like "we've paid for it, let's use it" more than any artistic or scenaristic decision.

In the end it leaves an impression of not really have been thought through to the end and they just gave up half way.