It's Randor's birthday, and he announces that he will be exploring the Plain of Perpetua with Adam and Teela. Skeletor has a plan to attack Adam by awakening the Behemoth that lives in the Labyrinth of Perpetua (under the Plain of Perpetua), put under a sleep spell by the Ancients thousands of years beforehand. Evil-Lyn makes it sound like Teela is calling for help inside the labyrinth. Adam, without his sword, rushes in, but is chased by the Behemoth. Teela and Orko are attacked, but eventually Orko manages to get the Sword of Power to Adam. He-Man closes the labyrinth's entrance, trapping Skeletor, Evil-Lyn, and the Behemoth inside.
(Yet more info about the Ancients taming ancient Eternia...)