The series is set in the fictional Beach City, which is in the state of Delmarva on the American East Coast. In Beach City, the Crystal Gems live in an ancient beachside temple, protecting the world from evil. Ageless alien warriors, they project female humanoid forms from magical gemstones that are the core of their being. The four Crystal Gems are Garnet, Amethyst, Pearl and Steven, a young half-human, half-Gem boy who inherited his gemstone from his mother, the Crystal Gems’ former leader Rose Quartz. As Steven tries to figure out his powers, he spends his days with his human father Greg, his friend Connie, other people in Beach City, or the other Gems, whether to help them save the world or just to hang out. He explores the abilities passed down to him by his mother, which include fusion — the ability of Gems to merge their bodies and abilities to form new and more powerful personalities.
The first season slowly reveals that the Crystal Gems are remnants of a once great interstellar civilization. Most of the locations they visit are ruins that were once of great importance to Gem culture, but have become derelict for millennia. The Gems are cut off from their homeworld, and it becomes apparent that the monsters and artifacts they encounter are Gems who can no longer maintain rational humanoid forms. In parallel, flashbacks related by Greg develop the history of Rose Quartz and her relationships. By the end of the first season, Steven learns that the Homeworld Gems intended to sterilize the Earth to incubate new Gems within it. Now, 5,000 years after Rose Quartz led the other Crystal Gems in a violent and apparently successful rebellion against these genocidal plans, the homeworld’s machinations once again extend towards the Earth with the arrival of two hostile envoys, Peridot and Jasper. In the second season, Peridot is forced to ally with the Crystal Gems to prevent Earth’s destruction by a Gem abomination growing in the planet’s core, while Jasper remains locked in a struggle with Lapis Lazuli, an errant Gem.