The Simpsons has occasionally teased the idea that Springfield has its own multiverse featuring alternate versions of the titular family. Season 32's "Treehouse of Horror XXXI" even featured multiple versions of Homer teaming up in a parody of Spider-Man: Into the Spider-Verse. But Homer isn't the first member of the Simpsons family to get their own multiversal storyline.
The Simpsons Super Spectacular #2 features the story "The League of Extraordinary Barts!" by Ian Boothby, John Delaney, Andrew Pepoy, Chris Ungar, and Karen Bates, which focuses on three versions of Bart Simpson from across parallel dimensions: Bartman, Stretch Dude, and Cupcake Kid. How were the trio united, and how did they save a parallel reality from their most nefarious multiversal counterpart?
Each of the trio is a superhero version of Bart from a different reality: Bartman (who appeared primarily in the Bongo Simpsons Comics) works with his sidekick Houseboy -- aka Milhouse -- to fight street-crime in Springfield using a variety of gadgets and tricks. Stretch Dude (introduced in Season 11's "Treehouse of Horror X") has the ability to stretch and shift his body, similar to Mr. Fantastic from Marvel or Plastic Man from DC Comics. The Cupcake Kid (introduced in Season 15's "Simple Simpson" before becoming a prominent character in The Simpsons Super Spectacular) is the only one who's a sidekick instead of his own hero, working alongside his world's version of Homer -- who maintained his work as the local superhero Pie Man -- with the pair using baked goods as weapons in their war on crime.
All three are drawn to another dimension's Springfield, which has been reduced to rubble at the hands of a destructive villain. The trio was brought to this dimension by this reality's version of Sideshow Bob, who reveals himself to be heroic in this timeline and a member of the local resistance. He called upon the three to help deal with their master -- revealing that it's actually this world's version of Bart. In this reality, Bart didn't have the core of goodness that defines the others, keeping them good-hearted pranksters. Instead, this dark Bart decided to stop being an underachiever in evil, and kidnapped Prof. Frink's beloved lab monkey Mr. Bananas -- forcing the scientist to construct devices he could use to conquer first Springfield, and then the world.
This evil Bart turns the local bullies into his elite police, keeps Principal Skinner as personal entertainment and even forces Frink to create a device that changes people into birds -- turning it on his family and trapping them in a cage as pets. The evil Bart offers the three heroes the chance to join him, with only Cupcake Kid seeming to agree to the idea. But in reality, it's just a distraction to give Stretch Dude and Bartman time to escape and find Mr. Bananas. With his beloved monkey returned to him safely, Frink turns on the evil Bart and helps the trio of heroes defeat him. The natural effects of the reality-bending device finally wear off, sending all three of the heroic Barts back to their native realities just as the crowds of enslaved Springfield citizens storm the castle.
It's a light-hearted and fun adventure, with all three distinct versions of Bart being similar but also retaining elements that make them unique. Stretch Dude's enhanced abilities make him a key player for the group, while Bartman's tech and confidence allow him to be surprisingly destructive when he needs to be. Finally, Cupcake Kid has to survive his missions by using his wits, with the hero's quick-thinking giving the trio the chance to escape and overcome their dark counterpart. They even promise to team-up again down the line, prompting Lisa to hint that there may even a multiversal team of incarnations of herself out there, something that the Barts (predictably) find hilarious.