Experiment 390 was the 390th genetic experiment created by Jumba with Hämsterviel's funding. He was designed to leave slippery slime trails for enemies to trip on. 390 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 390's pod landing in a sleeping tourist's belly button at a hotel.
The man later awoke and flicked 390's pod into the nearby swimming pool, activating the experiment.
In "627", 390 was tracked down and captured in a containment orb by Stitch. However, 627 was able to abduct 390 from Stitch and hand the experiment over to Gantu. 390, named Slimy, was later seen hanging out at Gantu's ship, where he shook his slime all over Reuben, much to the latter's disgust.
Slimy 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 Slimy, 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.
Slimy 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.
Slimy made a cameo appearance in the Stitch! anime as an image on a computer screen.
- After Reuben and Sparky, Slimy is tied with Digger for the most appearances of experiments that debuted in Stitch! The Movie. Both appeared in two different episodes of Lilo & Stitch: The Series as well as Leroy & Stitch. Though Slimy briefly appeared on a computer screen in the Stitch! anime.
- Slimy is one of three known limbless experiments, the other two being Zap and Cloudy.
- Slimy's pod color is red.
- Strangely, Slimy seems to change from dark green in Stitch! The Movie and light green in the series to dull blue and dirty clear. It is unknown how or why this happens, but he is shown twice in Leroy & Stitch like this, and his original green version did not appear. It is possible that it was a general recolor in order to fill out the group of experiments to make it appear as if there were more experiments than were actually designed.
- Slimy also changes in size from large in Stitch! The Movie to small in "627". It is unknown if there was a reason for this or just a change in animation style.