The Sorceress of Castle Grayskull is woken one night by a mysterious magic sword that leads her to a glowing portal known as a ‘Time Gate’. Recognizing the sword as the ‘Sword of Protection’, the Sorceress summons Prince Adam and Cringer the tiger to Castle Grayskull and sends them through the portal to find the person destined to possess the sword. Finding themselves in the otherdimensional world of Etheria, Adam and Cringer stop at an inn for lunch and discover Etheria is ruled by an evil intergalactic army known as the Horde. When some Hordesmen soldiers cause trouble in the inn, Adam stands up to them and gets into a fight which he wins with the help of an archer named Bow, who tells Adam that he and his friend Kowl are members of the ‘Great Rebellion’.
As word of the fight reaches Hordak, leader of the Horde, Bow and Kowl take Adam and Cringer to the Rebellion’s base in the Whispering Woods. They meet the other Rebels, including their leader Princess Glimmer, tree people the Twiggetts and Madame Razz, the comically inept witch, who arrives on her talking Broom to reveal that the Horde are threatening to enslave the villagers unless the Rebels responsible for the fight in the inn give themselves up. Bow is willing to do this, but Adam and Glimmer convince the group that they should fight back to save the villagers instead. As the Horde, led by Force Captain Adora, start taking away the villagers they are attacked by the Rebels, aided by Adam and Cringer in their secret identities as He-Man and Battle Cat. He-Man confronts Adora and the Sword of Protection glows in her presence, revealing that she is the one he’s looking for – unfortunately this distraction allows the Horde to knock He-Man out and capture him. Madame Razz uses divination to discover that the Horde have taken He-Man to their prison complex on Beast Island and the Rebels head there to attempt a rescue. In the prison, Adora interrogates He-Man and agrees that the sword seems to be meant for her, to which He-Man retorts that he is to give it to someone who serves good rather than evil. As it turns out Adora thinks the Rebels are evil and the Horde the rightful, benevolent rulers of Etheria, although she admits to not knowing much about life outside the Horde’s base. When He-Man dares her to see for herself what life on Etheria is really like, Adora says she’ll think about it. The Rebels arrive on Beast Island and manage to get into the prison to find He-Man, only to get captured and imprisoned themselves. Luckily, Kowl manages to elude capture and frees He-Man, who then frees the others and destroys the prison. In the meanwhile, Adora has ventured into the towns outside the Fright Zone and sees first-hand the cruelties Etheria’s citizens are forced to endure at the hands of the Horde.
As Hordak and Shadow Weaver discuss how He-Man is too powerful a threat to ignore, they are confronted by Adora wielding the Sword of Protection. She has discovered how cruel the Horde truly are, but Shadow Weaver enchants Adora into a mystic sleep that will make her forget what she learned and takes the sword, planning to learn its secrets. Later, Hordak shows the Horde his latest weapon the Magna-Beam, a willpower-fueled transporter that will allow him to send the entire Rebel base into exile forever. However, none of the Horde’s captives have sufficient willpower to fully charge the machine. He-Man sneaks into the Horde base looking for Adora, but Adora once again thinks he’s the villain and arrests him. Hordak then has He-Man put in the Magna-Beam to charge it overnight. Late that night, Adora has nightmares about He-Man’s fate and hears a voice calling her name. She discovers the Sorceress talking to her through the Sword of Protection and convinces her to help He-Man, whom the Sorceress reveals is not only the good guy but also Adora’s twin brother. Instructed to hold aloft the sword and say “For the Honour of Grayskull!”, Adora is transformed into the superpowered She-Ra, Princess of Power. After she rescues and revives He-Man, the pair destroy the Magna-Beam and make their getaway on Adora’s horse Spirit, who in She-Ra’s presence is transformed into a talking winged unicorn named Swift Wind.
She-Ra then reveals that she is He-Man’s sister, leaving him confused as he’s sure he doesn’t have a sister. When She-Ra explains that she was told by the ‘woman in the sword’, He-Man uses the Sword of Protection to contact the Sorceress and she explains everything: When Adam and Adora were born to King Randor and Queen Marlena, Eternia was invaded by the Horde. Unable to defeat the combined might of the Eternian army and the magic of Castle Grayskull, Hordak plotted to demoralize them by kidnapping the newborn royals, aided by his favorite pupil (and He-Man’s future archenemy) Skeletor. Although the kidnapping was interrupted by Man-At-Arms, Hordak escaped with Adora and ultimately fled through a Time Gate. The Sorceress was unable to discover which dimension Hordak took Adora to, so she cast a spell that wiped all memory of Adora from the people of Eternia except for herself, Man-At-Arms, King Randor and Queen Marlena. Thus Adam was raised unaware of his sister’s existence. Convinced by the Sorceress’ story, He-Man happily accepts She-Ra as his sister. Returning to the Rebel camp as Adam and Adora, the Rebellion accept Adora into their ranks after learning that Adora was mind-controlled into serving the Horde. The Rebels have also discovered that Queen Angella, rightful ruler of the kingdom of Bright Moon, is being held prisoner on nearby Talon Mountain, so Adam and Adora volunteer to rescue her. As He-Man and She-Ra, they defeat Queen Angella’s jailer Hunga the Harpy Queen and reunite her with her people (including her daughter Glimmer).
Adam takes Adora back to Eternia to reunite with their parents, but Hordak has found out that Adora is with the Rebels and pursues them through the Time Gate. Finding himself in Eternia, Hordak goes to his old base on Snake Mountain and discovers that Skeletor is now the principal villain of Eternia. Skeletor is not pleased to see his old mentor, but upon learning that Hordak is after Adora agrees to help him to be rid of him. Magically disguised as cooks and with Hordak hidden inside a giant cake, Skeletor and his henchmen manage to infiltrate the royal palace and kidnap Adora. As Man-At-Arms, Teela and He-Man reassure the distraught king and queen that they will save Adora, Skeletor betrays Hordak and forces him back to Etheria, planning to ransom Adora himself. However, Adora manages to outwit her captors and, reclaiming her sword, deals with the villains as She-Ra before running into the rescue party. As He-Man introduces She-Ra to the others and helps her to convince them that Adora is safe, Skeletor is left bemoaning “A female He-Man! This is the worst day of my life!”
Adora decides to return to Etheria to aid the Rebellion, a decision accepted by her family, and the Sorceress sends Adora and Spirit back to Etheria, telling them they can use the Sword of Protection to summon aid from Eternia should they ever need it. Adam and Cringer tag along, offering to “help [Adora] get the Rebellion off to a big start”. As He-Man and She-Ra, the twins help the Rebels liberate Bright Moon, learning more about She-Ra’s powers in the process (including using empathy to communicate with the wild animals of the Whispering Woods and healing Swift Wind when he’s shot by the Horde). He-Man and Battle Cat then return to Eternia, while She-Ra and Swift Wind resolve to stay until all of Etheria is free.