At Celestia's request, Twilight volunteers herself and Shining Armor to watch over the Overlook Hotel through the winter. However, it's not long before isolation and solitude begins to wear at the two of them, and that's before they start seeing things that can't possibly be there.

She should probably have told Shining that the hotel has a reputation for driving its caretakers a little crazy. After all, all work and no play makes for dull ponies...