Here's the gist of it:
a) Evil villains can destroy the Hogfather forever if they can get everyone to stop believing in him.
2) No Hogfather means no Hogswatch*!
*The Christmas and Yuletime celebration of Discworld
The Hogfather is out of action. The presents need to be deliver.
Who could step in to take the Hogfather's place?
Why, Death, of course.
COWER, BRIEF MORTALS. HO. HO. HO.
Death enlists the help of his butler, Albert, to help him deliver presents on Hogswatchnight* to all the good little boys and girls in Ankh-Morpork. He even makes an appearance at a mall, listens to what the kiddies want for Hogswatch, and urges each little boy and girl to be good.
THAT IS, AFTER ALL, PART OF THE ARRANGEMENT.
*Christmas Eve. You've caught on by now, right?
But everything isn't all sugar canes and mistletoe. The sinister forces at work to destroy the Hogfather will do the same to Death, if they can.
Fortunately, Death's granddaughter, Susan*, figures that Something Is Up and brings her wits, bravery, common sense, and poker** to the rescue.
*Yes, Death's granddaughter. It's complicated?
**A fireplace poker, not a card game.
And for our entertainment, the Death of Rats*** joins the adventure too. Along with his steed, a raven.
***Want to know how the Death of Rats came to be? You have another reason to read Reaper Man.
If Death and Susan can't stop the villains from stealing everyone's belief and destroying the Hogfather, there will be no happy faces on Hogswatch morning.
Oh...and the sun might not rise either.