In the Lego universe, populated by anthropomorphic minifigures, villain Lord Business steals a super-weapon named the "Kragle" and blinds Vitruvius, a wizard who attempted to stop Business. He speaks of a prophecy before being kicked by Lord Business into a lava lake.

Eight-and-a-half years later, in the city of Bricksburg controlled by Lord Business (now employing the title of "President Business"), ordinary construction worker Emmet Brickowski notices a mysterious woman at his construction site and falls underground attempting to accost the trespasser. Emmet sees a mysterious piece and receives epileptic visions after touching it. He awakens in the custody of Good Cop/Bad Cop, Lord Business's lieutenant, with the piece, referred to by Bad Cop as "the Piece of Resistance", glued on his back.

The woman (named Wyldstyle) rescues Emmet from an attempt to disconnect the piece from him and brings him to Vitruvius in the Old West. Emmet learns Wyldstyle and Vitruvius are "Master Builders",^[11] capable of building anything whatsoever from their imagination without requiring instructions. Wyldstyle explains that Business wants to use the Kragle (a tube of Krazy Glue) to freeze the world into his own personal vision of orderly perfection. Though disappointed to find Emmet is not a Master Builder, Wyldstyle and Vitruvius are convinced of his potential when he recalls visions of a deity referred to as "The Man Upstairs" and Emmet's possession of the Piece of Resistance, which is revealed as capable of disarming the Kragle.

Emmet, Wyldstyle and Vitruvius evade Bad Cop's forces with the aid of Batman, Wyldstyle's boyfriend. They visit the hidden realm of Cloud Cuckoo Land, where they meet Unikitty. They attend a council of Master Builders, who are unimpressed with Emmet and refuse to fight Business. Bad Cop's forces invade Cloud Cuckoo Land, having placed a tracking device on Emmet. Emmet, Wyldstyle, Batman, Vitruvius, Unikitty and an astronaut named Benny manage to escape, as Cloud Cuckoo Land is destroyed. Emmet devises a plan to infiltrate Business's office tower and disarm the Kragle with aid from his friends, and the robot pirate MetalBeard. The plan nearly succeeds, but the group is captured and imprisoned in the Think Tank, where the Master Builders are forced to make instructions. Vitruvius resists but is decapitated by Business, who throws the Piece of Resistance into the abyss outside the tower, sets a self-destruct protocol and leaves everyone to die, including Bad Cop.

A dying Vitruvius reveals he made up the prophecy. He soon reappears to Emmet as a ghost and reveals that believing is what makes one the Special. Strapped to the self-destruct mechanism's battery, Emmet jumps into the abyss and severs the connection, saving his friends. Inspired by Emmet's sacrifice, Wyldstyle rallies the Lego people across the universe to build weapons and vehicles to fight Lord Business's army of Micro Managers while Bad Cop allies with the Master Builders.