An asteroid gets passed Primus' planetary shield. As the lasers from the Starship Eternia doesn't phase it, He-Man heads out to intercept it and deflects it into the sea. Master Sebrian discovers that the asteroid is causing trouble for the underwater city of Serus where Hydron is from. Learning form Hydron's sister Aquata about Serus' flooding, the Galactic Guardians work to seal the crack. Soon, they must deal with the living asteroid Keto who plans to drain Primus of its energy.