Open Grave (2013)
A man wakes up in a deep pit of dead bodies, with no knowledge of how he got there, or even who he is. He joins a small group of people with the same mysterious amnesia, as they each try to figure out their own identities, who they can trust, and what on earth is going on.
The film starts like 28 Days Later (but with more clothes) as the main protagonist finds himself in a world where something is very wrong, but has no idea how or why. The viewer learns and makes discoveries along with him, as clues (and clever misdirections) are slowly and tantalisingly revealed.
There is a sinister, fetid smell of dread throughout, with music and sound effects that heighten the atmosphere and the ick factor. On top of that, there's the creepy isolation of the forest location, and some deliciously gruesome imagery (corpses bound to posts with barbed wire, and so on). Sharlto Copley is compelling to watch as the central character, who you're never quite sure whether to like or be afraid of. In fact, despite the lack of big names in the cast, all the lead actors give strong performances here.
Open Grave is very much a mystery movie within a horror setting (reminding me of The Cube without the sci-fi elements). The plot takes several turns before the truth is finally revealed, keeping you guessing right up to the end. I've deliberately said very little about the story itself, as I think this is a film that's best watched knowing as little as possible beforehand. But I will say that I was completely drawn in, and had a great time trying to work out the puzzle. And I certainly didn't see the ending coming.