Experiment 128 was the 128th genetic experiment created by Jumba with Hämsterviel's funding. He was designed to turn entire civilizations into harmless insects. 128 and the other first 624 experiments were deactivated and smuggled to Earth by Jumba during his mission to capture Experiment 626.
All of the experiment pods were released and scattered across the island of Kauai, with 128's pod landing in the middle of a farm crop.
When the crop was being irrigated via sprinklers, 128's pod was sprinkled, activating the experiment. 128 then took flight and traversed the island of Kauai aimlessly.
128 was soon attacked by an eagle, but defeated it by turning it into a dragonfly. 128 then fluttered off laughing, but accidentally flew into Lilo's jar. Unaware that 128 was actually an experiment, Lilo named him Bugby and took him home, where she placed Bugby in her makeshift bug city, Bugopolis.
When Stitch grabbed Bugby and attempted to eat him, the latter turned Stitch into an ant in self-defense.
Later, when Lilo noticed Bugby in her bedroom outside of Bugopolis and went to check on the other bugs, Bugby turned Lilo into a ladybug.
Jumba and Pleakley shortly after went up to Lilo's room to bring her gravel muffins (which Jumba deemed inedible), and the two discovered Bugby flying loose, arousing Pleakley's interest. When Jumba then tried to warn Pleakley about Bugby, the latter turned Jumba into a stink bug and Pleakley into a praying mantis.
Later, the four befriended an arachnid, caterpillar and dung beetle, which they believed Stitch was evil since he ate insects like them. However, when the three bugs were in danger of insect-hating Nani and the exterminator, Stitch rescued them and eventually had a change of heart about eating bugs.
After a few setbacks, including a tussle with an aggressive wild cat, the gang used a special machine on Jumba's ship to revert to normal (and enlarge the three real bugs to human size). When the exterminator showed up, thinking the vermin crawled into here, the three giant bugs told the exterminator off, scaring him away.
Bugby then attacked, reverting the giant insects to their original size. However, Lilo and Stitch were able to recapture Bugby in Lilo's jar before he could turn her, Stitch, Jumba and Pleakley back into bugs. After this experience, Stitch no longer felt the desire to eat insects and arachnids.
Shortly after, Jumba reprogrammed Bugby to turn inanimate objects into bugs instead of people, and sent Bugby onto the same farm where he was first activated, where he turned rocks into aphid-eating ladybugs.
The first 624 experiments, including Bugby, were rounded up by Leroy and taken to a stadium to be destroyed. However, Lilo, Stitch, Jumba, Pleakley, Reuben and Gantu arrived before the experiments could be destroyed.
Bugby participated in the following battle between the experiments and the Leroy clones probably by turning attacking Leroys into insects.
However, the Leroys soon gained the upper hand in the battle, but were defeated when Lilo, Stitch, Reuben and several other experiments performed the song "Aloha `Oe", which caused the Leroy army to shut down due to the original Leroy's fail-safe.
- Bugby's pod color is purple.
- Bugby is described by the experiment computer screen as, "Experiment 128. Primary function: Bug morpher. Turn civilizations into harmless insects."
- In his episode, Bugby's eyes are purple, but in Leroy & Stitch, they are black.
- Bugby is one of the few experiments to not appear in the group photo at the end of Leroy & Stitch.