The main character is the incompetent and cynical wizard Rincewind, who involuntarily finds himself as a guide to the naive tourist Twoflower. After they are forced to flee from the city of Ankh-Morpork, they meet two barbarians, Bravd and Weasel, parodies of Fritz Leiber's fantasy heroes Fafhrd and the Gray Mouser.

Both characters also have a narrow escape from Bel-Shamharoth — a monster inspired by H. P. Lovecraft, who wrote many stories in a universe where unspeakable Evil lives, and where Ancient Gods (with unpronounceable names) play games with the lives of mortals. It is interesting to note that Lovecraft also wrote a story called The Colour out of Space, about an indescribable, unnatural colour.

The number eight is important on the Discworld, being closely associated with magic Wizards avoid saying the number out loud, and Octarine is the Disc's eighth colour, the colour of magic as described in the title. This is reflected in the title of the French translation of the book La Huitième Couleur — (lit: The Eighth Colour).

After a visit to the Wyrmberg, an upside-down mountain which is home to dragons that only exist in the imagination, and nearly falling off the edge of the Disc, their journey leads them to the country of Krull, perched on the very edge of the Discworld.

Their story is continued in the succeeding Discworld novel, The Light Fantastic.