Experiment 609 was the 609th genetic experiment created by Jumba with Hämsterviel's funding. He was designed to shoot heat rays from the large bulb on his forehead. 609 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 split into four.
Heat engaged one of the clones in a duel. The clone picked up the couch to use as a weapon, but as he only possessed one-fourth of the original Stitch's strength, picking it up was a struggle for him. Seeing his chance, Heat used his heat ray to carve a hole in the bottom of the couch, and due to the sudden imbalance, it collapsed on top of the clone. Gantu then picked the latter up and placed him in a containment orb.
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 Heat into a hundred replicas. However, as Heat'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 Heat clones into the original.
In "Remmy", Heat was one of the experiments in Lilo's dream.
Heat 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 Heat, 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.
Heat participated in the following battle between the experiments and the Leroy clones.
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.
Heat made an appearance in a later episode of the Stitch! anime along with Swapper. Heat was mistaken for Splodyhead by Jumba and Pleakley. Swapper switched Stitch and Hiroman's bodies. Then, when Heat attacked, Hiroman required to figure out how to use Stitch's abilities to win. Heat later appeared in the special Stitch and the Planet of Sand.
- Heat is one of the ten known experiments named by someone other than Lilo, the other nine being Gigi, Elastico, Yaarp, Hammerface, Thresher, Plasmoid, Morpholomew, Shrink, and Leroy.
- Even though we saw Heat in pod form along with Plasmoid, Thresher, and Hammerface, he was the only one activated onscreen.
- In the English dub of the Stitch! anime episode "Swapper 2.0", Jumba erroneously refers to Heat as "Splodyhead".