The Discworld floats through space on the back of four elephants that, in turn, stand on the back of the Great A'Tuin, the cosmic turtle. It only gets weirder from here.
On the Disc is the ancient city of Ankh-Morpork, and it's here that a man named Twoflower begins his journey as the world's first tourist. At the Broken Drum tavern, he meets a failed wizard named Rincewind, who knows exactly one spell and exactly nothing about what it does. Twoflower hires Rincewind as his guide, wanting to see the heroes, wonders, and majesty (read: cutthroats, dangers, and utter shabbiness) of the city.
Rincewind thinks this is such a great idea that he attempts to flee the city. He's stopped at the city gate and brought before the Patrician, though, who offers Rincewind a classic Hobson's choice: either act as Twoflower's guide and protector and most likely die in the process, or accept the Patrician's punishment and wish he had died in the process.
Choosing the former, Rincewind brings Twoflower on a tour of Ankh-Morpork's sites: the taverns, brothels, and merchant quarters. Twoflower is kidnapped by a group of thieves lead by Ymor, though, and brought back to the Broken Drum.
With the help of Twoflower's Luggage, Rincewind mounts a rescue for the tourist, only to find himself in the middle of brawl between the Thieves' Guild, the Assassins' Guild, and the Traders and Merchants' Guild, all of whom want Twoflower for their own purposes.
Rincewind and Twoflower escape the city of Ankh-Morpork as it burns down around them (a disaster that is only marginally their fault), and the tour of Discworld continues.
Rincewind and Twoflower travel to the forest. Thanks to a game between the gods, a troll is put in their path leading to an unfortunate series of events involving a mother bear, a pack of wolves, a poisonous snake, and the personification of Death itself. The two travelers are separated: Rincewind ends up in a tree serving as the home for some dryads, while Twoflower finds his way to the Temple of Bel-Shamharoth, the Soul Render.
Rincewind is found guilty of injuring the tree during his mad dash to escape the wolves, but before the dryads can execute him, they open a portal to show him Twoflower's fate in the Temple. Using it to escape, Rincewind crashes into Twoflower, begging him not to speak the number eight (an unlucky number on Discworld and the number of the Soul Eater).
Since all paths of the Temple lead to the center, they find themselves in the lair of Bel-Shamharoth with a fellow adventurer, Hrun the Barbarian. Rincewind explains to Hrun not to say the number eight but doesn't warn his sword, Kring, a sword with the magical ability to talk incessantly.
The utterance of eight awakens Bel-Shamharoth, and a desperate struggle for survival and sanity ensues. The flash from Twoflower's iconograph sends the otherwise invulnerable beast back to its inner-dimensional nether-realm.
And the tour of Discworld continues with Hrun in tow, who is paid to protect the tourist with pictures of himself striking heroic poses.
The travelers make it to Wyrmberg, an upside-down mountain housing a warrior nation of dragonriders. Awesome.
Hrun and Twoflower are captured by dragonriders on the orders of Liessa, who wants to marry Hrun and use his brainless brute force to claim the throne of Wyrmberg. Meanwhile, Rincewind and Kring—mostly Kring—defeat a dragonrider and force him to bring them into the dragon's lair. Inside Wyrmberg, Rincewind is mistaken for the hero he doesn't want to be and duels Lio!rt, Liessa's brother.
Elsewhere, Hrun accepts Liessa's offer to battle her brothers for an immense treasure, and Twoflower accidently taps into Wyrmberg's magical powers to create his very own dragon. Using the dragon, he rescues Rincewind right when he loses his duel.
Hrun defeats both of Liessa's brothers, and Liessa offers to marry him. Hrun accepts just as Rincewind and Twoflower swoop in on the dragon to "save" him from his terrible fate—thanks, guys—and as they fly away from Wyrmberg, the power to create the dragon dissipates, and it disappears. Hrun is saved by Liessa, but Rincewind and Twoflower plummet to their death.
They are saved from Death thanks to some inner-dimensional shenanigans that pop them over to our Roundworld for a quick visit before they finish their decent into the Discworld ocean.
Rincewind and Twoflower travel to the very edge of the Discworld—it being a disc, the edge is quite real and quite literal. They are caught by the Circumfence, a net circling much of the Disc's edge, and picked up by a sea troll named Tethis.
Tethis takes them to his house located on a crag right on the edge. He explains that he's actually from a whole different world, but he accidently fell off and travelled through space until landing on the Discworld. He also explains to them that they are now officially slaves of the Kingdom of Krull.
Rincewind and Twoflower are picked up by Krull's hydrophobic wizards, whose powers result from their massive loathing of water. They are taken to Krull where they are told they won't be slaves after all—instead, they'll receive the honor of being sacrifices.
With the help of the Lady, who's still doing awfully well at messing up Fate's plans, Rincewind and Twoflower escape. They board the Potent Voyager, a spaceship designed to go on a mission to determine the sex of the Great A'Tuin. At the same time, Luggage, with Tethis in tow, finally catches up with them, causing havoc throughout Krull, resulting in massive damage and some incredible special effects.
The Potent Voyager launches, but Rincewind is unable to board. He's instead launched off the world to land on a precariously perched rim-tree. There, he has a brief conversation with Scrofula, where Rincewind simply refuses to die regardless of how helpful it would be to the faux-death.
Rincewind falls into space to join the Potent Voyager and discover what waits in the rest of Creation. Probably more of the same, but you never know.