Due to an apparent shortage of dark matter, which is used to power starships, fuel prices have begun to rise, prompting Professor Farnsworth to prohibit flying the ship unless absolutely necessary, as it would otherwise waste fuel. However, Leela has begun to develop anger issues, leading her to enter the ship in a demolition derby. As punishment, Leela is forced to wear a shock collar that activates whenever she has a violent thought; later in the film, though, she ... more
Due to an apparent shortage of dark matter, which is used to power starships, fuel prices have begun to rise, prompting Professor Farnsworth to prohibit flying the ship unless absolutely necessary, as it would otherwise waste fuel. However, Leela has begun to develop anger issues, leading her to enter the ship in a demolition derby. As punishment, Leela is forced to wear a shock collar that activates whenever she has a violent thought; later in the film, though, she begins to enjoy the shocks and eventually develops an urge to kill things. Meanwhile, Bender feels left out when he sees Cubert and Dwight playing Dungeons & Dragons and can't play because robots are not installed with imaginations. Trying as hard as he can, Bender manages to imagine himself as a medieval knight and joins the game, naming himself "Titanius Anglesmith." However, since he was never meant to have an imagination, Bender gradually believes himself to be a real knight in the magical world of "Cornwood" and, after being subdued during a rampage through the city, is sent to the HAL Institute for Criminally Insane Robots.
Farnsworth learns that Mom, who controls the dark matter industry, has amassed a record-breaking fortune despite investing heavily on her dark matter mine in Alaska; as it turns out, the "shortage" is a ploy Mom is using for profit. Farnsworth reveals that when he was still working for Mom many years ago, dark matter was a completely useless substance. While experimenting with it, he inadvertently produced two crystals: an energy crystal that turned dark matter into fuel, which Mom took for herself, and an opposite "anti-backwards matter" crystal, which Farnsworth kept hidden. Farnsworth says that should the two crystals ever meet, they would render dark matter useless once more. The anti-backwards crystal, now believed to be missing, is currently being used as a 12-sided die in the kids' game. Learning this, Mom sends her sons Walt, Larry, and Igner to retrieve it, but Farnsworth manages to find it first, and he, Fry, and Leela take it to Mom's dark matter mine to put an end to her plans.
Infiltrating the mine, the crew learns that it is actually a farm where thousands of Nibblonians are being held captive for their ability to excrete unlimited amounts of dark matter. The three are spotted by Igner who, having overheard a startling secret Mom revealed about him to Walt and Larry, helps them escape his brothers. They confront Mom with the energy crystal and try to bring the two crystals together, but Farnsworth is forced to swallow it to keep it safe from Mom, so Mom threatens to remove it with laxatives. While this happens, the crystals cause all dark matter in existence to resonate oddly, including the matter Bender had stored within himself (having collected it as "treasure" earlier during his insane crusade), causing it to bring the realm Cornwood to life as an alternate universe and sending him, the whole Planet Express crew, Mom and her sons into it.
Only Fry and Leela are shown emerging in Cornwood with the anti-backwards crystal; not only that, but Leela has transformed into a centaur, and no one else there seems to have any memories of the real world. They are met by an armored Bender in his "Titanius" persona, who names his friends "Frydo" and "Leegola," respectively. The three are pursued by Mom's sons, now named Waltazar (Walt), Larius (Larry), and Ignus (Igner); in the ensuing confrontation, Frydo accidently drops the anti-backwards crystal which rolls like a die and lands on a 7, banishing their pursuers to a swamp. They meet up with the Great Wizard Greyfarn (Farnsworth) and learn that the anti-backwards crystal, or the "Die of Power," was created by the evil sorceress Momon (Mom), who hopes to re-obtain its immense power. To stop her, the heroes must travel to Momon's lair in the Geysers of Gygax and cast it into the molten plastic from where it was molded, which is the only way to destroy it. At the start of their quest, the group encounters an intersexual centaur named Hermaphrodite (Hermes) who promotes his/her people's nonviolent lifestyle despite being master archers, much to Leegola's chagrin due to her newfound sadistic pleasures. Joining them on their quest is Gynecaladriel (Amy), Queen of the "Water Nymphos" with the ability to seduce anyone (making out with Greyfarn, Titanius, and even Leegola over the course of the film).
Traversing the Cave of Hopelessness, home of the dreaded Tunneling Horror, the group is stopped by Zoidberg, now a giant lobster-like creature. Leegola assumes Zoidberg to be the Tunneling Horror and brutally slays him in cold blood, only to learn he is not the same monster. Wracked with guilt over killing an innocent creature, Leegola abandons her friends just as the real Tunneling Horror, a giant worm, emerges, but not before accidentally decapitating Zoidberg by casting her sword aside. Frydo rolls the Die of Power which lands on a 3, which turns Frydo into a giant and allows him to defeat the Tunneling Horror. Frydo begins to grow obsessed over the Die and becomes a mad, impish creature. After a botched attempt to murder his friends, Frydo flees.
As Leegola learns the ways of peace from the centaurs, the others travel to Wipe Castle to amass an army against Momon, only to learn Roberto, as its insane king, had sent it on a pointless suicide mission, leaving the heroes to defend the alone as Waltazar and Larius lead a blockade against them. Learning that her friends are in trouble, Leegola regains her will to fight and rallies the centaurs into an army, swiftly breaking the blockade. Frydo, meanwhile, is met by Zoidberg's still-living head, having sprout tentacles and survived on its own after being cut off, who offers to help Frydo destroy the Die of Power. Reaching the Geysers of Gygax, Frydo finds he is too attached to the Die to destroy it, so Zoidberg bites him to help him let it go. As Frydo drops the Die, Momon turns into a dragon to stop him. The Die rolls on a 12 which turns Frydo into a dragon as well, and he battles Momon as his friends arrive. Ignus, in his attempt to help the heroes flee from danger, reveals that, having overheard from Momon, he is Greyfarn's son, meaning Igner is Farnsworth's son in the real world. Overwhelmed, Greyfarn falls from the heroes' perch and lands on Zoidberg who, in his temptation, had stolen the Die of Power, allowing Momon to obtain it and causing Cornwood to collapse on itself.
Everyone is suddenly sent back to the real world, regaining their original forms and memories. With the anti-crystal back in his stomach and Mom threatening to remove it again, Farnsworth asks to hug his son Igner, instinctively determining Igner had swallowed Mom's energy crystal in defiance. Their embrace brings the crystals in their stomachs in close enough range to each other, rendering dark matter useless and effectively ruining Mom's dark matter empire. As a temporary substitute for fuel, the Planet Express crew uses the captive Nibblonians like sled dogs to pull their ship home.
The Planet Express crew get trapped in a fantasy world.