Experiment 617 was the 617th genetic experiment created by Jumba with Hämsterviel's funding. He was designed to shoot plasma balls from his scorpion-like tail. 617 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.
Back at his ship, Gantu trained and conditioned Hammerface, Heat, Thresher, and Plasmoid by using Experiment 625 for target practice, much to his indignity. The four henchmen experiments were then led by Gantu in an attack on the Pelekais' house, where their boss ordered them to defeat and capture Stitch and his replicas after seeing that he had been divided into four.
While his teammates each battled one of the clones, Plasmoid engaged the original Stitch in a duel. Plasmoid attacked by firing plasma balls at Stitch, only for the latter to dodge them all. The scorpion-like experiment then fired a plasma blast at an overhead lamp. Stitch taunted Plasmoid, saying he missed, but this was proven wrong as the lamp then fell on Stitch's head, knocking him out. Gantu finished the battle by placing Stitch in a containment orb along with his clones.
After capturing Stitch and his duplicates, the five returned to Gantu's ship for a victory feast (courtesy of Reuben), but were interrupted by Lilo, Pleakley, and Dupe. After Lilo tricked Gantu into using Dupe's cloning ability, Gantu used Dupe to clone Plasmoid into a hundred replicas. However, as Plasmoid's power was divided among them, they all became complete weaklings, and thus were easily defeated.
Presumably after this, they were all sent to Hämsterviel who was somehow able to re-merge the weakling Plasmoid clones into the original.
Plasmoid reappeared in "Snafu", and was freed along with Gantu's other captive experiments by Lilo, Stitch, and several other experiments in a rescue mission.
The first 624 experiments, including Plasmoid, 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.
Plasmoid participated in the following battle between the experiments and the Leroy clones by firing plasma blasts at Leroys.
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.
Plasmoid has appeared twice in the Stitch! anime, both times with Splodyhead. In the first appearance, he and Splodyhead fought against Stitch when he was hypnotized by Swirly into fighting for Hämsterviel. In the second appearance, he and Splodyhead were modified by Hämsterviel to attack Stitch. Plasmoid was given a new ability to vacuum up objects and using them as projectiles.
- Plasmoid is one of the ten known experiments named by someone other than Lilo, the other nine being Gigi, Elastico, Yaarp, Hammerface, Heat, Thresher, Morpholomew, Shrink, and Leroy.
- Plasmoid's pod color is blue.
- Plasmoid is described by the experiment computer screen as, "Experiment 617. Primary function: Plasma shooter."
- Plasmoid appears in the online game Lilo & Stitch: Alien Interception along with Tank, Dupe, Clyde, Yang, and Hämsterviel.