Teppic, prince of a country that closely resembles Ancient Egypt has been sent to Ankh-Morpork to be trained as an assassin. This is a practical career choice for someone who will one day reign as Pharaoh in the desert kingdom of Djelibeybi.
News of the old Pharaoh's death arrives in Ankh-Morpork via magical phenomena such as vegetation sprouting wherever Teppic steps. He makes his way back to Djelibeybi, filled with new-fangled ideas on how to move his country out of the Millennium of the Pyramid and into the Century of the Fruit Bat.
But the pyramids themselves are formidable enemies to progress. They trap time. Prince Teppic's father is dead, but he is still sentient and horribly aware that he is being turned into a mummy. He has high hopes that his son will end all of this death-worshiping nonsense, but Teppic is guilt-tripped by the high priest into building his father the biggest pyramid of all.
When the young king finally realizes that the mad, old high priest is the real ruler of Djelibeybi, he dons his black assassin's outfit and sets about rescuing beautiful handmaidens, thwarting the sacred crocodiles and their priests, and confronting Djelibeybi's ancient animal-headed gods.
Not bad for a kid who started out by letting the grass grow under his feet.
"Pyramids" is not part of a Discworld miniseries, but stands alone as a satire on religion, ancestor worship, ancient Greek philosophers, mathematicians, piratical merchants (another great career opportunity for a graduate of the Assassins Guild) and anything else Pratchett felt like taking a swing at.
This is ancient Egypt and Classical Greece as seen through the eyes of a mathematical genius who happens to be a camel, a young assassin who happens to be a king, and a mummy who would rather not spend the rest of his afterlife in Discworld's biggest pyramid.