Skeletor uncovers a stone tablet which reveals the secret of the Ancients, and he constructs a device to tap the power of Eternia. Meanwhile, Teela and Stratos, disturbed about reports that Mer-Man and his army are moving towards Grayskull, send for He-Man. Later, Skeletor encloses the castle in an unseen bubble of power and Mer-Man's army uses zero-energy weapons to crystalize the River Doom, forming a dam. Further upstream the river floods and the waters lap around Grayskull, full of creatures. In Grayskull, all the vehicles and power weapons are rendered useless. Stratos flies to ask help from Man-At-Arms, who has recently created a solar cannon. The Wind Raider lowers the cannon onto Grayskull, whereupon Teela fires a blast at the dam, bursting it and washing away the machine which created the power shield. Upon defeat, Skeletor destroys the tablet.