Trama

Within the Lego Universe, Batman (Will Arnett) continues to protect Gotham City and fight crime, but during his latest mission to stop the Joker (Zach Galifianakis) from destroying the city, he hurts his arch-rival's feelings by telling him he is not as important in his life as he thinks he is, leading Joker to seek the ultimate revenge on him. (Joker had high jacked a plane transporting military explosives and had used that to create a powerful bomb which he attached to the nuclear reactor of Gotham's power station. His plan was to detonate the bomb and create a chain reaction that would destabilize the plates on which Gotham is built, leading to destruction of the city. As usual Batman stops Joker, but when Joker says Batman is obsessed with him, Batman simply says he doesn't believe in relationships).

The following day, Batman's alter ego, Bruce Wayne, attends the city's winter gala, which is celebrating both the retirement of Commissioner Gordon (Héctor Elizondo) and the ascension of his daughter Barbara (Rosario Dawson) as the city's new police commissioner, only to be infuriated by Barbara's plans to restructure the police to function without the need of Batman. Without warning, Joker crashes the party with the city's other villains, all of whom surrender, with the exception of Harley Quinn (Jenny Slate), who disappears during the confusion. Once Joker and all the other villains surrender, the city feels that this is the end of crime in Gotham and tells Batman to retire and enjoy time with his family. Batman suddenly realizes that he has no family and lives all alone.

Suspecting his arch-rival is up to something, Batman plots to steal Superman's (Channing Tatum) Phantom Zone Projector, a device that can banish anyone to the Phantom Zone - a prison housing some of the most dangerous villains in the Lego multiverse. Just as he plans his heist of the device, Alfred Pennyworth (Batman's butler) (Ralph Fiennes) intervenes and advises him to take charge of Dick Grayson (Michael Cera), whom Bruce unwittingly adopted as his ward during the gala. Reluctantly agreeing to do so, Batman fosters Dick as Robin, whereupon the pair successfully recover the Projector from Superman's home, Fortress of Solitude (where he has been having the 57th annual justice league party without Batman), and break into Arkham Asylum to send Joker to the Phantom Zone. Annoyed at his recklessness, and suspecting his arch-rival wanted to be sent there, Barbara locks up the pair.

While the Projector is being seized as evidence, Harley steals it back as part of Joker's plan and frees him, allowing him to return to Gotham with all the villains that had been trapped in the Phantom Zone, including: The Wicked Witch of the West (Poison Ivy) (Riki Lindhome), Count Dracula, a Tyrannosaurus Rex and a Velociraptor, Voldemort (Eddie Izzard), Sauron (Jemaine Clement), and King Kong (Seth Green). After terrorizing Gotham city, Joker attacks the Wayne Mano, after learning from Sauron that Batcave is built under Wayne Manor.

Realizing that the city does need him, Barbara releases Batman and Robin and teams up with them and Alfred to stop the Joker. Although Batman finds himself able to trust and rely on his new team (together they take on Joker and his villains, & manage to destroy Sauron's eye), but upon reaching Wayne Island, he chooses to send them away, rather than losing them like his parents.

Upon facing him alone, Joker express belief that Batman is incapable of change (when again Batman refuses to say "I Hate you" to Joker) and zaps him to the Phantom Zone, before stealing the Batcave's stash of confiscated bombs. Arriving in the Zone, Batman witnesses the harm his selfishness has caused to everyone and slowly accepts his greatest fear. Making a deal with the Zone's gatekeeper, Phyllis (Ellie Kemper), to be allowed back to Gotham in order to retrieve the Zone's escaped prisoners, Batman arrives in time to save his team-mates, apologizing to them for his actions, and requesting their help to stop Joker.

Realizing that Joker plans to use the explosives he took to destroy the city's Energy Facility, thus causing the city to come apart and be destroyed, Batman turns Barbara into Batgirl, before he and his team, along with the city's other villains, who felt neglected by Joker, manage to successfully send back the escaped villains to the Phantom Zone. However, the group fail to stop Joker's bombs going off, as the explosion begins to tear the city apart at the plates beneath it. Knowing this was his fault, Batman reluctantly convinces Joker that he is the true reason for being the hero he is, before they, their friends and allies, and the city's inhabitants, chain-link themselves together and pull the plates back together, saving the city.

With the city saved, Batman prepares to be taken back into the Phantom Zone to fulfill his bargain, only to be rejected by Phyllis, who chooses to let him remain after seeing how much he had changed in order to save everyone. Batman allows Joker and the rest of his rogues' gallery to temporarily escape, with the confidence that whenever they return, they will be no match for the combined team of himself, Robin, Batgirl, and Alfred.