I won't ruin too much of the story, but from what little I've played of it, basically you're some random guy who has imbibed some potion to take away his memory.
You stumble your way through the dark to find a note from yourself, to yourself prior to the memory erasing which asks you to murder an old man.
It also warns of some evil entity which is stalking the player.
The game does a pretty good job of simulating an individual with a pretty severe psychological disorder.
Little things, including the dark drive you crazy, and you have to do whatever you can to prevent yourself from slipping into complete dementia.