Primus has been taken over by the Evil Mutants and the Zeps. As He-Man and Mara have the key to the shield around Primus, Skeletor has captured Master Sebrian, Hydron, and Flipshot and sends B.H., Quakke, and Staghorn to find He-Man. Meanwhile, the Scientists of Primus work to find what makes the Zeps assume their monstrous forms.