A stranger has taken the place of the Hogfather. Well, not exactly a stranger. He's actually pretty well known. Even the laugh is wrong. The switch has been arranged by the Auditors, mysterious superbeings who want our universe to be a collection of rocks swinging in curves through space.
Life is messy.
Somebody has to rescue the real Hogfather before this morbid impostor tracks soot on the world's carpets. And they'd better hurry. The bogus Hogfather is asking the wrong questions. Which cuts no ice with Death. Everything starts somewhere, although many physicists disagree.