"Dark Knight Court" is the sixteenth episode of Season 24.
When Bart is falsely accused of playing a prank on the school band which he didn't do, Lisa defends him in student court; Mr. Burns is vengeance - he is the night (well, the early evening) - he is Fruit Bat Man.
Springfield is having an Easter celebration, where the school band is about to play a song. However, hundreds of eggs are inadvertently launched from the instruments, landing on the crowd and ruining their clothes. The Springfieldians wonder who could have done this and discover Bart laughing at the mess. Everyone immediately concludes and thinking that Bart pulled the prank and ruined Easter, although Bart denies committing the prank. As the townspeople chase Bart through the streets, Mr. Burns and Smithers take shelter in the Android's Dungeon. Burns observes the superhero comics within and recalls his younger years of reading comic books, which came to an abrupt end when his father, Clifford, bought out the comic book shop and burned it down. Burns then offers to buy the entire stock, and when Comic Book Guy requests a payment of "the speed of light expressed in dollars", Burns tells Smithers to "just give [Comic Book Guy] Faraday's Constant". At home, the family accuses Bart of pulling the prank as he continues to deny any involvement that he's innocent and didn't pulled the Easter prank. Suddenly, the family starts hearing ominous music coming from outside, where they find Mr. Largo and the other members of the band threatening to kill Bart. Desperate, Bart turns to Maggie to help him prove his innocence, as she is the only one Bart claims to never lie to. Maggie stares at her brother and slowly nods her head, confirming his innocence via the "Maggie test". Lisa, now convinced to help her brother, protests for a trial to the band. Mr. Largo refuses, so Homer creates a distraction by conducting the band to play a song, after which he receives a bouquet of flowers from Ned.
At Burns Manor, Burns takes a liking to an issue of "Detection Comics", featuring a Batman-like character. He informs Smithers that he shall become a crime fighter, and he names himself Fruit Bat Man. The same night, Mr. Burns battles crime in Springfield but is unable to realize that Smithers is faking the crimes to protect Burns.
As Lisa struggles to find a judge who would help their case, Grampa tells her and Bart a story of how he met Janet Reno in 1998, but the kids refuse to believe it. The following day, Bart's trial is held at Springfield Elementary and Superintendent Chalmers reveals the judge to be Reno, proving Grampa's story true. After her defense statements, Lisa makes a plea for clemency, but Reno denies the plea, summoning all of Bart's enemies (including Homer, Moe, Sideshow Bob, and Apu) to testify against him. At home, Lisa reads about Burns' deeds as Fruit Bat Man and makes a smug yet bold statement: "A Simpson never gives up" (to which the rest of the family laughs uproariously, including Maggie). Lisa then goes to Burns Manor to ask the billionaire for help. Burns refuses to help her and is later outraged when he discovers that Smithers faked the crimes. Smithers then tells Burns that Lisa's plea could have been the first time he actually helped someone.
The next day, Lisa discovers a small splatter on Groundskeeper Willie's kilt, which she and Marge deduce was created by hand. When they question him, Willie admits that he committed the prank because he and others from his birth country of Scotland all hate Easter. Willie destroys the kilt to remove the evidence, but Burns (as Fruit Bat Man) chases after Willie and eventually defeats him by stuffing a wad of money into his tractor's exhaust pipe. Lisa thanks Burns for helping her expose the true culprit to the school. Bart is thus cleared of all the charges, and later that night, he tries prank calling Moe's Tavern, where Moe weeps when he learns that his tormentor has been acquitted. As the closing credits roll, a trailer for "The Dependables" (a parody of Marvel's The Avengers) is shown, starring several elderly characters from the show who play poker against each other while dressed as superheroes.