With Mumm-Ra and Pryon defeated, Shadowmaster is summoned from the Shadow Realm to try and defeat the Thundercats.
His goal is to create a great eclipse and overtake Thundera with darkness.
At the Cats Lair, Panthro detects the fast approaching eclipse and Cheetara is dispatched to Sector 7 to see what is happening.
When Cheetara arrives she sees Shadowmaster standing in the pathway as his evil shadows cover the ground like a menacing, spreading oil. Shadowmaster holds a small gleam of light in his hand as he tells Cheetara to deliver a message of defeat to the Thundercats. Shadowmaster closes his hand, which extinguishes the light, and the shadows close in.
Cheetara arrives back at the Cat Lair and tells of Shadowmaster's plans. Lion-O summons all the Thundercats with his Sword of Omen.
The Thundercats face the shadows but find themselves very ineffective against them. The Thunderkittens are able to stave off some of the shadows using their flashbombs. However, they begin to run low on them.
Lynx-O detects where Shadowmaster is, and suggests that the only way to defeat the shadows is to destroy Shadowmaster himself.
Lion-O attacks Shadowmaster, and by using the Eye of Thundera destroys him. The eclipse subsides.
Back at the Pyramid, Mumm-Ra is summoned once again to destroy the Thundercats.