Death has been working for a very long time and he's starting to get lonely. He has a wizard to make him food and an adopted daughter to keep him company but he feels like he needs a break. Death goes in search of an apprentice and finds Mort, a clumsy young man. Death teaches Mort how to usher souls to the next world and other aspects of the business then leaves him to take over. What he doesn't anticipate is that Mort falls in love with a princess he's supposed to help die and instead saves her, altering the world. The world does not like being altered though and will do everything to set things straight again.
This isn't the first Discworld book I've read but I think it's my favourite one so far and one that has made the most sense. Given the chance, I would start with this book first as I think it introduces Discworld in the most gentle way.
There were some parts to this novel where Pratchett would get in to the intricacies of Discworld and I have to be honest I started glossing over. I was more interested in the characters and how they were going to get out of their trouble.
First Line: "This is the bright candlelit room where the life-timers are stored - shelf upon shelf of them, squat hourglasses, one for every living person, pouring their fine sand from the future in to the past."