After trying to keep silent about their unsanctioned explorations, WilyKat and WilyKit finally reveal their discovery of Mumm-Ra's Black Pyramid to the rest of the ThunderCats. Once he learns what the cubs found within its obsidian walls, Lion-O makes a fateful decision: He will go, alone, on a quest to confront Mumm-Ra in his place of power. If he can best the devil-priest in single combat, the ThunderCats' claim to their new home will be secured - but if he fails, the last survivors of Thundera may lose everything!