Plot summary
The episode begins with Steven (Zach Callison) waking up in the body of a Watermelon Steven, one of the sentient watermelons shaped like himself that he created in the first-season episode "Watermelon Steven". Steven sees that the Watermelon Stevens have created their own society on Mask Island, complete with homes, jobs and rituals. Steven's watermelon body is selected for one such ritual and led to the top of a cliff. Suddenly, Malachite (Kimberly Brooks as Jasper and Jennifer Paz as Lapis Lazuli) rises up from the ocean and devours him whole.
Steven wakes up at his family's barn in his own body and tells Garnet (Estelle), Amethyst (Michaela Dietz), and Pearl (Deedee Magno Hall) that Malachite is active. They tell Steven to stay put with Peridot (Shelby Rabara) while they go to the island to fight Malachite. Steven astral-projects into another Watermelon Steven to help them.
On Mask Island, the Crystal Gems fuse into Alexandrite (Rita Rani Ahuja), and she and Malachite engage in battle. During the fight, Alexandrite accidentally destroys the warp pad the Crystal Gems used to teleport to the island.
As Malachite gains the advantage, Steven rallies the frightened Watermelon Stevens, who have taken refuge in a cave. The Watermelon Stevens arm themselves and march into battle.
Malachite immobilizes Alexandrite in a block of ice. Before she can finish her off, the Watermelon Stevens begin their assault, managing to down and distract Malachite for long enough for Alexandrite to free herself. As Malachite begins smashing the Watermelon Stevens, Alexandrite reengages in the battle. She soon gains the upper hand and defeats Malachite, who de-fuses into her component Gems, Lapis and Jasper, both unconscious.
Alexandrite defuses herself back into an exhausted Garnet, Amethyst, and Pearl. They thank Steven and the Watermelon Stevens for their bravery and enjoy a brief moment of celebration; but suddenly, the island begins to tremble and a giant fissure forms across the beach. Amethyst manages to catch Lapis, but Jasper falls in. Garnet realizes that the earthquake portends the emergence of the Cluster, an underground geo-weapon that threatens to destroy the Earth, and tells Steven that he must drill to the Earth's core with Peridot to destroy the Cluster. As he begins to regain consciousness, the Gems remind Steven that they love him.
Credits
Adapted from Wikipedia contributors, Super Watermelon Island. Paragraphs and formatting adjusted; illustrations, reference markers, block quotations and non-story material omitted.