I have played through the demo and the only storyline to back you through this haunted trek through a castle is this:
You play as a guy named Daniel. Judging by his vocabulary and setting the game takes place in an Edgar Allen Poe-esque era where he attempts to cover up his mistakes by drinking a mysterious potion that erases his thoughts and memories. A memory-erasing potion per say?
Once he drinks it, you wake up in his place, wondering who, what, when, where, and how. He leaves a note to himself to Murder another man located in the castle once he has finished the drink, but doesn't explain why and only tells you to not dilly dally, for something is a miss...
Everything after that can only be explained in the demo.
