The Dollhouse — Hayden Thorne

A strange force is at work in a magnificent house filled with revelers. When midnight strikes to mark Christmas Day, guests find themselves in the wrong company. An old couple who despise each other sit together in a private, romantic corner. A frail young girl is surrounded by chaperones she dislikes. Servants are busy in the kitchen or serving their masters, though their minds are elsewhere. A young man proposes to a lady, in all appearances his perfect match, but his heart lies in a neglected musician who copes with his inexplicable isolation in the conservatory.

No one knows why this happens every time Christmas rolls around, and no amount of defiance on the guests' part ensures the preservation of their true partnering when midnight strikes again.

But the answer to the riddle is simpler than they think.