At the Royal Palace Man-At-Arms attempts to modify his Bashin' Beetle vehicle. Orko arrives and begins to interfere with his work. A sudden power surge surprises both Man-At-Arms and Orko. Man-At-Arms detects that the source of the power surge emanating from the Sea of Rakash.
At Snake Mountain Evil-Lyn explains to Skeletor that the Coridite crystal, long thought destroyed, still exists. Skeletor orders his warriors to retrieve the crystal. At the Sea of Rakash, Man-At-Arms begins his underwater search for the crystal while Prince Adam, Cringer and Teela remain on dry land. Meanwhile Skeletor deduces that Mer-Man has possession of the Coridite crystal.
Man-At-Arms arrives at the source of the recent power surge: Mer-Man's lair. Man-At-Arms confronts Mer-Man and the villain's warriors begin to attack. Man-At-Arms manages to grab the Coridite crystal, and hides as Evil-Lyn and the other villains make their way to Mer-Man's lair. Adam and Teela notice that Orko has vanished, much to Man-At-Arms aggravation as Orko accidentally reveals Man-At-Arms' location to the villains. Mer-Man is about to retrieve the crystal from Man-At-Arms, when his giant fish ally swallows Man-At-Arms and the Coridite crystal. Unexpectedly inside the giant fish the Coridite causes a mutation, making the creature extremely powerful and dangerous. Inside the creature the mutation causes its internal organs to attack Man-At-Arms. The creature attacks a small village close to the Sea of Rakash.
Teela summons the other Masters to the Sea of Rakash, only to learn from Orko that her father and the Coridite crystal are inside the giant fish. Adam and Cringer quickly leave the scene and transform into He-Man and Battle Cat. The Masters lead the creature away from the village. They lead the creature into a dense forest where Teela stands out in the open as bait for the creature. As the creature roars He-Man leaps into the creature's mouth.
Inside the giant fish Man-At-Arms is swept further into the creature's stomach, and a series of spiked walls form, one of which traps Man-At-Arms' leg. He-Man finds Man-At-Arms and the Coridite crystal, and passes it to Man-At-Arms. Both he and the crystal are lost when the creature's internal body acid takes him further down. Man-At-Arms resurfaces with the crystal. He-Man draws his sword to help pull his friend to safety, but surprisingly the Coridite crystal reacts with the sword and suddenly Castle Grayskull is revealed as the source of great power. The giant fish begins to make its way toward Castle Grayskull.
The Sorceress watches as the creature plows through the dense forestry. Man-At-Arms appears on Teela's Battle Hawk viewscreen and commands her not to let the creature reach Castle Grayskull, even if it means destroying the creature, thus killing him and He-Man. The Masters all stand ready in defense of Castle Grayskull. As the Masters are about the fire He-Man and Man-At-Arms emerge from the creature's mouth. He-Man knocks the creature into the abyss surrounding Castle Grayskull. The Coridite becomes unstable and Man-At-Arms warns He-Man that it is about to explode. He-Man decides to try and absorb the power through his sword. A massive explosion takes place and when the smoke is cleared all that can be seen is a stone statue of He-Man. The villains begin cheering thinking He-Man is no more, but the hero escapes his stone-like state.
At Snake Mountain Skeletor claims that as a result of Teela's acceptance to sacrifice her father in defense of Castle Grayskull he knows that something of great power must dwell within.
Synopsis by James Eatock