A man eats a single mushroom very, very slowly for the duration of the film. He is filmed (mostly in focus) with a stationary camera. A cat enters the screen at one point. The cat leaves for a while but returns. The man pets and strokes the cat all the while continuing to eat the mushroom. After the cat leaves the second time, the man continues to eat the mushroom until the film (and the mushroom) are finished.
Although IMDb warns that short descriptions of less than ten lines may not be adequately detailed, I believe that a longer description is probably not reasonably possible. I think I have included everything that bears mentioning.