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I saw this film about 10 years ago but I don’t specifically know how old it is. Here is everything I remember:
Attributes:
There are only 2 cast members. The story is narrated by the main character, this comprises almost all the dialogue as the 2 characters never meet (sort of).
Plot:
The film opens with a British soldier waking up in an unfamiliar bed in an unfamiliar house, all the windows and doors are sealed with a bright white light being cast into the house. The house contains a stocked kitchen, a piano, and a bookcase filled with books that have no writing on any of the pages. at a specific time during the day a strange zombie-like creature dressed in a tattered military uniform appears in the house with no obvious point of entry and attacks the main character. The main character overpowers and kills the creature and after exploring the house thoroughly eventually returns to bed. The main character dreams of blurry, vague images of tall grass and distant, muffled sounds. when he wakes the next morning he finds the house has reset itself with new food in the kitchen all all items returned to their former place. the zombie creature’s body is gone and at the exact same time as the previous day the creature re-appears and once again attacks the main character and once again is killed.
The next section of the film is a montage sequence where the main character gets so used to the repeating nature of the house that he has set up a trap to kill the creature and can spring the trap without paying much attention as he knows the exact timing of its attacks. One day the main character decides rather than killing the creature to capture and interrogate it instead, asking how it enters the house each day, what it is doing there, who it is and so on. It becomes clear that the creature either can’t or won’t talk. The main character leaves the creature tied up and goes to bed, barricading his door. This proves fruitless as he is woken by the creature mauling him to death and he wakes up the next morning in the bed with no blood but with healed scars on his abdomen.
Eventuality the main character takes to dismantling the house as much as he can each day and he finds a basement with an old military style trunk and uniform. Inside the trunk he finds a bottle of (presumably) whisky, some personal affects, and books like the ones in the bookcases upstairs but with diary entries inside from another soldier who at one point was trapped in the same house. He reads through the diaries as he continues his daily routine finding that the former occupant, after an immense amount of time, began taking his own life on a regular basis. No matter what method he tried the former occupant would always wake up in the same bed but with new scars each time, leaving him deformed and unable to draw, or play piano, or do any of the things that kept him sane. The main character begins to realize that the deformed zombie creature attacking him each day is the former occupant, having lost his mind. The end of the diary entries revel that any damage done in the basement of the house does not reset and any items left there remain there. The diary also reveals if you tunnel far enough through the basement you eventually find a buried door that once opened can either be stepped through or walked away from, however if you walk away it will never open again. that is how the former occupant became trapped for long enough to become zombified. The main character begins digging and find the door and sees on the other side himself, dying on a battlefield. He steps through and the film ends with him dying in the real world reveling that the dreams he’s been having were of this moment.

Answered question