One by one, the Lions get their tails kicked. It looks pretty hopeless. The Princess, though she crashed, is the only one left with a Lion that is still operable (though barely) by the time Haggar and Lotor arrive. Allura was spared because Haggar had the Robeast save her for Lotor. How kind! The Robeast splits open as if molting a skin, and the nearly identicle one that emerges is ordered to destroy the Castle.
Meanwhile, Coran tries to help the Princess get her Lion moving. While she's frantically trying to get going, the Robeast arrives at the Castle and breaks in through the moat. The Castle quickly starts to flood. Nanny and Coran run for cover in the Castle control room with the Robeast knocking ominously at the door.
Allura does get her Lion going (by figuring out a code called the Violent Pentagram, which Alfor programmed into all the Lions to get them going again in just such an emergency), but Coran assumes she'll go free the other Lions first leaving them to be Robeast dinner. She does manage to free all the Lions but Keith's, but just when they think they're winning Haggar reminds them about the townsfolk and what kind of danger they're in. When things go from bad to worse at the Castle, Allura leaves the others to guard the town while she tries to rescue Coran and Nanny. Allura, after saving the space mice from Coba, goes to her father's tomb leaving Coran to face off with the Robeast with a cannon. Her father gives her a special key to place in the lock of the insignia crest.
She opts to fight off the robeast while Cheddar delivers the key to the right place. Much magical contrivance ensues and all the Lions are freed, not to mention the shell robeast explodes. Now they're free to deal with the Robeast in the castle. Once Voltron is formed, a big old battle erupts. It's pretty lengthy, but I'm sure you can guess how it ends. The Robeast defeated and with Lotor once again slinking off in defeat, the Arusians go about repairing their cities and the castle. Allura gives a nice speech on the swellness of Average Joe.