See now I was trying to remember how She-Ra fit into the whole He-Man universe.
Minor lore dump:
Eternia and Etheria are two separate worlds in two different dimensions. Eternia, home of He-Man and Skeletor, is the world we're most familiar with.
King Randor and Queen Marlena had twins they named Adam and Adora during an attempted invasion by the intergalactic Horde. The Horde's forces were led by Hordak. Hordak kidnapped infant Adora. Man-at-Arms captured Hordak's student (and Randor's half-brother) Skeletor. Skeletor ratted out Hordak and told Man-at-Arms where to find him. Before Man-at-Arms could rescue Adora, Hordak escaped through a dimensional portal to Etheria.
Years later, Hordak had managed to take over Etheria and Adora had become a Force Captain in his army. The Sorceress begins having dreams about the events during the Horde invasion and wakes to find the Sword of Protection floating above her. She follows the sword through Grayskull to a new portal. The Sorceress summons He-Man to take the Sword of Protection through the portal and seek out its owner. He-Man travels to Etheria, gets captured, meets Adora (they have no idea they're related), and decides to give her the sword.
The sword allows Adora to break free of Shadow Weaver's mind control, and she learns about her past from a spiritual projection of the Sorceress. She becomes She-Ra for the first time. She frees He-Man, He-Man starts breaking Hordak's toys, She-Ra jumps on Spriit, turning him into Swift Wind, and flies off to warn the Rebellion that Hordak is coming. When She-Ra returns to help He-Man finish breaking Hordak's toys, she tells him they're twins. Mic drop.
They summon the Sorceress's spirit through the Sword of Protection and get the whole story about what happened when they were infants. Emotional reunion. He-Man and She-Ra free the Queen, and their work here on Etheria apparently being finished, they return to Eternia for a big family reunion. Hordak (sneaky bastard) sees them go through the portal and follows.
The royal family throws a party. Hordak and Skeletor crash the party and kidnap Adora (again). She tricks Beast Man into giving her sword back and defeats Skeletor. Hordak turns tail and flees back to Etheria. Adora tells her parents the she has to return to free Etheria from Horde oppression. Crying ensues. Adora returns home.