Megamind (Will Ferrell) is a super-intelligent alien who -- at the age of eight days old -- is sent to Earth from his home planet during a time of crisis. However, he lands in The Prison for the Criminally Gifted, thanks to another infant alien similarly sent from a different planet who eventually becomes a superhero known as Metro Man (Brad Pitt), defender of the fictional Metro City. As a result, Megamind is raised as a social deviant, who is neglected for his destructive intellect, with his fish-like sidekick Minion (David Cross) as his only friend. Realizing that he enjoys causing trouble, and jealous of all of the attention Metro Man receives, Megamind becomes a supervillain to rival Metro Man, though he is thwarted on a regular basis. During one of his many attempts to defeat Metro Man, Megamind kidnaps news reporter Roxanne Ritchi (Tina Fey) and uses her as bait to lure Metro Man into the city observatory where Megamind intends to kill him with a death ray. The plan appears flawed enough for Metro Man to escape, but he proclaims that the inside of the observatory is lined with copper, which he says drains his power. Metro Man is then seemingly killed by the death ray. Megamind is initially happy that he has defeated his archenemy, but soon grows depressed over the fact that without someone to fight, his life as a supervillain no longer has any purpose.
Megamind talks with Roxanne while disguised as a museum curator named Bernard and is inspired to create a new superhero to fight using a serum of Metro Man's DNA. Hoping to uncover any evil plans Megamind has for the city, Roxanne breaks into Megamind's lair and fights with him over a syringe gun containing the serum, which is accidentally fired and injected into Roxanne's cameraman Hal Stewart (Jonah Hill). Megamind manipulates Hal into becoming a superhero named Tighten, but at the same time falls in love with Roxanne and begins dating her while disguised as Bernard. Tighten attempts to use his powers to woo Roxanne, but is quickly spurned and grows jealous of the Bernard-disguised Megamind. Megamind soon begins having second thoughts about being a villain, finding more enjoyment pursuing a relationship with Roxanne. He then gets in a fight with Minion and the two friends have a falling out. After Roxanne rejects him when his identity is accidentally exposed, Megamind follows through with his plan to fight Tighten. By this time, however, Tighten has decided he would rather be a supervillain since in his view, being a superhero is too much hard work. Unlike Metro Man, Tighten has no qualms with outright killing Megamind and he is forced to flee for his life. With nobody to stop him, Tighten begins wreaking havoc in Metro City.
Noticing how one of his copper-based attacks during the fight had no effect on Tighten, Megamind convinces Roxanne to search for clues on his weakness in Metro Man's old hideout. There they find Metro Man alive and learn that he faked his death (and his weakness to copper) so that he could retire from being a superhero. Megamind and Roxanne try to persuade Metro Man to defeat Tighten, but he refuses and instead Megamind turns himself in. When Tighten kidnaps Roxanne, however, Megamind is broken out of prison by Minion and goes to save her. Megamind defeats Tighten by draining Metro Man's DNA from his body, stripping him of his powers, and Hal ends up in Megamind's former prison cell while Megamind is embraced by Roxanne and Metro City as its new defender. Megamind begins to enjoy his new reputation as a superhero while a disguised Metro Man silently congratulates him.