What is the main conflict in The Light Fantastic by Terry Pratchett?

The Light Fantastic is the second book in Terry Pratchett's Disc World fantasy series. It follows the adventures of the failed wizard, Rincewind, and his tourist friend, Twoflower. Rincewind has the eighth spell from the famous Octavo book stuck in his head. Unfortunately, the wizards need the spell to save the world from destruction.