He lives a mundane daily life; he does not leave his bedroom apartment—he works from home and has food delivered. In this mundanity, he finds himself curious and then entertained by the vividity of his dreams. After all, they are the only times in which he feels anything—a vast juxtaposition to his daily monotony.
Realizing this, he gradually develops a growing fascination and then obsession with his dreams. Slowly, he begins to remember more of them. In doing so, he is both in awe and horror at what the unconscious mind—his mind—is capable of. This progresses towards a deep incongruence in the man's identity. He was not the man he thought he was— in his dreams, at least.
In his dreams, he acts primally, without morals or ethics. In essence, he is the uncurated version of man, free from any societal expectations. He attempts to justify all the atrocities he commits in his dreams, saying they were not real people—yet he still praises himself for any good deeds in his dreams.
Eventually, the line between his waking world and dream life falls, and the two spill over into each other. He no longer knows what is real. He cannot remember if the day prior had been lived or dreamt, nor can he remember who he is—what he is. More tragically, he can now not escape the nightmares that chase him, even in his waking life—the guilt for what he did, even if he hasn't done anything real.
It all ends with him looking at himself in the mirror. It isn't himself who he sees.
