Why pretend all genres are created equal? Edmund Wilson couldn’t stomach mysteries, and said as much in “Who Cares Who Killed Roger Ackroyd?” In that spirit of disclosure: As soon as a pointy wizard hat shows up in a movie, I’m done. And The Colour of Magic is lousy with wizard hats. It lays its scene in author Terry Pratchett’s Discworld universe, subject of 25 years’ labor and a bookshelf-full of mythos. The International Cabal of Dorks, the 21st-century demographic of choice, can natter among themselves as to whether hack Vadim Jean has transcribed faithfully (and the rest of us can wonder why this tossbag is still working after Jiminy Glick in Lalawood. What incriminating photos does he have?). To one unsteeped in Pratchett lore, this celluloid spin-off—released as a two-part telefilm in the U.K.—is unwatchable, and nothing more. Rincewind (David Jason), a novice sorcerer well into retirement age, plays tour guide for an oblivious out-of-towner (Sean Astin) visiting the Kingdom of Ankh-Morpork. This involves trying to extract himself from whatever mortal peril his charge sightsees them into (chintzy CGI dragons, Belle Époque space exploration, Death). Spamalot-storybook-satirical “humour” should’ve been stopped in customs.