Experiment 303 was the 303rd genetic experiment created by Jumba with Hämsterviel's funding. He was designed to erase his victims' memories. 303 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.
303's pod was used as a ping-pong ball by Keoni and another player until it got whacked off the table and into a little boy's sand bucket. Thinking the pod was a marble, the boy put it on top of his sand castle. A small wave then splashed over the castle, activating 303, which caused the boy to flee the beach in terror.
303, named Amnesio, then proceeded to erase people's memories, including a married couple whose wife clashed with her husband after losing her memory. Meanwhile, Lilo and Stitch handed out homemade invitations to the former's birthday party.
Gantu later tracked Amnesio down, but found it very difficult to catch the latter while he was airborne, as he managed to successfully evade all of Gantu's plasma blasts.
Gantu and Amnesio's presence captured the attention of Lilo and Stitch. Grabbing a butterfly net, Stitch climbed a telephone pole to try to get Amnesio, but when the latter perched on a power line, he was captured in a containment orb by Gantu. Lilo, Stitch and Gantu were soon fighting over the captive experiment, which ended with Amnesio wiping all three of their memories, leaving the trio dazed.
Plagued with amnesia, Lilo and Gantu decided to team up and figure out what to do with the captured experiment. Stitch, however, reverted to his original programming, while Lilo and Gantu assumed he was an escaped convict.
With their memories erased, Lilo and Gantu (dubbed Martha and Lenny) went after Stitch and did recreational activities together at some point. They later ran into Jumba and Pleakley, where Amnesio wiped both of their memories, but not before they explained to Lilo and Gantu exactly who was on whose team.
Lilo and Stitch then decided to team up to get their memories back. Lilo wanted Gantu to come along, but the latter was unsure of himself. After Lilo and Stitch discovered how to reverse Amnesio's effects, Gantu set the experiment free. Lilo was devastated, even though Gantu stated that he didn't want his memory back.
Luckily, Stitch was able to use his telescopic vision to locate Amnesio, and the three chased him into a factory. Despite Gantu's protests, Lilo undid Amnesio's effects by yelling out, "ʻOhana!" Although everyone got their memories back, Gantu managed to recapture Amnesio after a long pursuit and take off with him.
Amnesio was successfully sent to an impatient Hämsterviel, but the former ended up erasing Hämsterviel's memory also. Gantu misleadingly described him as a dangerous criminal who had been captured by Ace Jackson (Gantu), while admiring a pottery jar that read "World's Greatest Cop," which Lilo had made Gantu earlier.
Amnesio reappeared in "Snafu", and was freed along with Gantu's other captive experiments by Lilo, Stitch, and several other experiments in a rescue mission.
Later, the first 624 experiments, including Amnesio, 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.
Amnesio participated in the following battle between the experiments and the Leroy clones by erasing Leroys' memories.
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 ʻOe", which caused the Leroy army to shut down due to the original Leroy's fail-safe.
Amnesio made an appearance in the Stitch! anime where he used his powers on BooGoo, Stitch, Jumba, Gantu, Reuben and Hämsterviel. It is revealed that Amnesio holds the stored memories in pouches under his wings. If his victims ingest the pouch with their memories in it, everyone's memory will be fully restored. Only Amnesio knows whose pouch belongs to whom. Also, it is revealed by Dr. Hamsterviel that he used Angel's song on few of the experiments; Amnesio's powers are used on Stitch, as the song reverts Amnesio to his original programming.
- Amnesio was first revealed in a DVD bonus feature of Stitch! The Movie in the experiment gallery.
- When Amnesio flies around in several instances of his episode, the song "Flight of the Bumblebee" plays.
- The password which reverses Amnesio's effects is "ʻohana", a word that Jumba didn't know of until well after Amnesio's creation (unless he made the adjustment after arriving on Earth).
- The password could also be "family", but it works in any language.
- Amnesio's pod color is yellow.
- Amnesio is described by the experiment computer screen as, "Experiment 303. Primary function: Erases memories."
- Amnesio appeared on the DVD cover of Leroy & Stitch, along with Lilo, Stitch, Jumba, Pleakley, Doubledip, Fibber, Drowsy, Woody, Shredder, Ploot and Felix, with Leroy flying above in a one-man space shuttle.
- Amnesio's head appears in the online game Jumba's Lab.