Wormhole is the twenty-fifth episode of Stitch!-The Mischievous Alien's Great Adventure.
The episode begins with Stitch and Yuna in their room looking at photographs of themselves, using a new holographic projector (presumably invented by Jumba). They're enjoying them until they're interrupted by Tigerlily, who throws boxes inside and sets up a treadmill for herself inside Yuna's room. Yuna looks with dismay at the photos her cousin messed up, and this, combined with Tigerlily's indifference to her takeover, makes Yuna wish she wasn't related to her and runs outside. Stitch tries to follow, but quickly loses her.
While running and desiring to get away from Tigerlily, Yuna comes across another one of Stitch's cousins that has come to the island: Experiment 272 AKA Wormhole. The purple caterpillar decides to open a wormhole in this spot and does so by levitating, curling into a ball and rolling in midair. Yuna asks him what a wormhole is and he answers that it's a portal to another universe. He then proceeds to explain that long ago (perhaps shortly after his creation), Jumba unpleasantly learned how nice he turned out to be and threw his favorite slipper at him out of anger, only for it to miss and fly through a wormhole he had opened up, disappearing into an unknown dimension. Wormhole finishes by saying that he blames himself for the incident and has ever since been making wormholes and searching alternate universes in an endless quest to recover his creator's lost footwear. Yuna eagerly asks to come with him, wanting to escape her selfish cousin Tigerlily. Wormhole advises against it, but seeing how insistent Yuna is, he reluctantly agrees. Stitch finally catches up just as Yuna steps into Wormhole's wormhole. He tries to stop her and grab her back, but is ultimately pulled in as well.
Yuna and Stitch arrive in the parallel universe in the middle of an abandoned city. Wormhole begins wandering around searching for Jumba's slipper while Yuna and Stitch suddenly look to the sky to see an airborne battle between Gantu (in his ship) and the alternate universe counterpart of Stitch (wearing a jetpack). Stitch battles and wins against Gantu by stealing his blaster, tricking him into severly damaging his own ship, and finally throwing him into the distance. Yuna and the reformed Stitch cheer at his victory until he pulls out a laser blaster and begins shooting and damaging buildings. Yuna angrily yells at him to stop, only for him to point his blaster at her and fire, the reformed Stitch pushing her out of the way just in time. He makes his way up to his doppleganger to confront him, and when Yuna calls out his name, Stitch's counterpart says "Stitch" is a stupid name. The reformed Stitch tells him that it's his name too, but his counterpart denies it, saying that he's Experiment 626, "the ruler of the universe".
During their fight, a big chunk falls off the building they're on toward Yuna. Stitch quickly climbs back down and catches it, but it's heavy even for him. He asks his counterpart for help, but 626 scoffs at him and leaves both Stitch and Yuna to be squashed. This is about to happen until a large crane lifts the building piece up. The driver gets out and reveals himself to be the parallel version of Jumba. Stitch and Yuna are happy to see him until he captures them with a sticky substance from a blaster.
Jumba takes them to his home where he lives with alternate Pleakley. He leaves Yuna tied and gagged on a couch and straps Stitch to an autopsy table, ready to dissect him. Pleakley takes Yuna's gag off and is surprised that she knows his name. Yuna pleads him to stop Jumba from dissecting Stitch. Pleakley threatens to sing if Jumba doesn't stop and he complies. Yuna and Stitch discuss their story over a meal cooked by Pleakley, who is as bad a chef in this dimension as he is in the other one. Jumba and Pleakley are marveled at how different Stitch is than 626, the former telling them that he does good deeds and shows him the Counter his Jumba made. Jumba is shocked that his counterpart doesn't seem to be as evil as him and Pleakley explains that Jumba's the most wanted criminal in all the galaxy. Yuna offers to show them photographs, but finds that she dropped the holographic projector sometime after arriving. Shortly after, the wall is suddenly blown apart to reveal 626. Jumba's upset at him for wrecking the house, but 626 is even more upset to see Stitch there. After rejecting Pleakley's cooking, he leaves to destroy the city some more.
At sundown, Yuna and Stitch look over the city from the rooftop. Yuna wonders out loud if there's an alternate counterpart of her in this dimension, and if they'll ever meet. She and Stitch then lament that 626 doesn't have an ohana like they do. Meanwhile, 626 is rampaging through the already-decimated city and proclaims his power out loud; however, he's met with nothing but an echo. He stumbles over the dropped projector and accidently activates it, showing pictures of Stitch having a good time with his friends. Yuna and Stitch go to bed while 626 sits alone.
The next morning, Yuna and Stitch are woken by their Jumba calling on the Good Deed Tracker, made possible by having planted a tracking device in Wormhole. He tells them that they only have twenty minutes to go back through the portal before it closes, trapping them in the other universe forever. He feeds Wormhole's coordinates into the tracker so they can find him and he can lead them home. Unfortunately, Wormhole refuses to leave without finding Jumba's slipper first. Stitch sees 626 flying by overhead and insists on attempting to rehabilitate him despite Yuna's protests.
Stitch meets up with 626 again and despite Stitch's claims that he can have an ohana, he starts fighting him again. Jumba and Pleakley encourage Yuna to come home, even if it means leaving Stitch behind, but she refuses to do so. Tigerlily then comes on and orders Yuna to come back. Yuna says she doesn't care about her, but her cousin argues, saying that she's been trying to be a role model for her and that they're ohana no matter what. Yuna, touched by her words, promises to come home, though when the connection's closed, Tigerlily says to herself that she was lying.
Stitch and 626 are still fighting, the latter winning because the former doesn't want to hurt himself. Yuna comes between them, but Stitch jumps in front of her and is promptly knocked down by 626. Before he can finish off his reformed self, Yuna tells him why Stitch won't fight him: he's ohana; to prove this point, Yuna hugs 626. He struggles for a second, but accepts the hug. Just then, Gantu flies overhead in a new ship and mistakes Stitch for 626. He pulls out a giant blaster and fires, but 626 jumps in front of Stitch and takes the hit, falling unconscious. Stitch chases Gantu off again by throwing a rock into the blaster, the subsequent explosion reducing his ship to a propeller-topped bicycle. Jumba then arrives in a hovercraft and takes the three back to his lab.