Over 6,000 years ago an alien race known as Gems tried to colonize the planet Earth. However their actions were unsuccessful as a group of Gems known as the Crystal Gems fought against their Homeworld to protect Earth. After a thousand years of war the Crystal Gems won, and Homeworld Gems left Earth alone. However it was won at a great cost. A once large are of Gems was reduced to just 4 Gems left on Earth; Garnet, Amethyst, Pearl, and their leader: Rose Quartz. While Gem activity was secede on Earth, there where still dozens of dangerous Gem artifacts left on Earth and the Crystal Gems protected Earth from them. They also protected Earth from other alien species that threatened the Earth. In modern times a group of humans and aliens know as the Plumbers protected the Earth from extraterrestrial threats. While they knew about the Gem race they didn't know about the failed colonization, the war, or the Gem artifacts left on Earth. They never knew about any Gem activity on Earth, including the Crystal Gems. In fact the Crystal Gems stayed as far away from the Plumbers as possible and have remained undetected, until now.