Lilo, Stitch, Jumba, Gantu and 625 are in pursuit of Experiment 515: the deforestator. The chase leads into town where 515 shreds everything in sight. Stitch corners him in an alley, defeats him, and rehabilitates him in record time. Gantu catches up to capture 515, but then retreats after 515 shreds his clothes along with 625's fur, leaving him in his underwear. Afterwards, both Lilo and Jumba complement on Stitch's performance in catching Experiments, which eventually gets to his head and becomes egotistical. When he behaves rudely at dinner and covers Jumba in cranberry sauce, he decides to teach him a lesson by creating a new experiment: 627. More powerful than Stitch with none of his weaknesses, has twenty different powers of other Experiments and cannot be turned to good.
When Pleakley discovers what Jumba has done, he tries to go and tell only for Jumba to threaten him to be quiet otherwise he would create another Experiment with a giant finger that can poke out his giant eye. At the same time, 627 escapes from containment and runs off into the forest. Meanwhile, after getting wrapped up in another argument over their recent failure to capture an Experiment, 625 goes out into the woods to have a sandwich. There he meets 627 and gives him to Gantu as his new sidekick, so he can stay at home and do nothing like he always does. Gantu is impressed with 627, especially when he demonstrates his ability to shoot plasma at 625.
The next day, Lilo and Stitch are at a farmyard where they are about to capture Experiment 603: a living laser bolt. Stitch is still being egotistical, even while he successfully captures 603. Just then Gantu shows up with 627, and takes 603 from them. Stitch challenges 627 and easily gets beaten. They return home with news of their encounter to Jumba and Pleakley. While looking through Jumba's Computer to determine which Experiment it was, Jumba accidentally confesses to 627's creation. Stitch refuses to accept that 627 is more superior than him, and over the next few days he competes against his newest "cousin" in hunting Experiments. 627 successfully captures many experiments and Stitch ends up getting relentlessly bullied and tormented by him. After a while, Stitch admits defeat.
Meanwhile, things for 625 are no better. Since 627 became Gantu's new sidekick, he has been abused and forced to clean around the ship. Gantu has also decided to use 627 to conquer the galaxy by himself, even quitting his job of working for Dr. Hämsterviel. When 625 gets blast by 627 and slips, he discovers his weakness is laughter when he laughs uncontrollable over his misfortunes. Because he has had enough of 627's abuse, he calls Lilo and informs her of what he has discovered. Jumba clarifies that he gave 627 extra everything, including an extra large sense of humor. With 625's help, Lilo and Stitch lure Gantu and 627 to the middle of town for a final battle. Stitch defeats 627 by making a fool of himself during their battle and using the latter's abnormally large sense of humor against him. Eventually, 627 laughs himself into unconsciousness, and since he cannot be turned to good Lilo dehydrates him back into a pod using a home food dehydrator she ordered from a TV commercial earlier.
Afterwards, Stitch has learned his lesson and is no longer egotistical. Meanwhile, however, Jumba is shown to have made an Experiment 628 and stores the pod away on his ship.
- Moral: If you want to feel good about yourself, be humble and don't let success go to your head.
- 627's creation was oddly foreshadowed at the end of Stitch! The Movie.
- 627 is the only experiment in the series that was never rehabilitated or given an actual name.
- Ironically, although Jumba said it was impossible to deactivate 627, Jumba was able to deactivate the other 625 experiments that were created before 627, excluding Stitch.
- When Stitch taunts 627, he stands on his head with his two hands flat on the ground and shakes his rear end, a callback to the first film. However, Stitch's butt is slightly overscale in this episode.
- 627's manner of defeat is similar to the Toon Patrol from Who Framed Roger Rabbit, though while most of the Toon Patrol die from laughter, 627 just passes out long enough to be dehydrated.
- The ending of the episode foreshadows Leroy & Stitch.