A Ghost and Four Keys
Worried that the Space Explorers might succeed in reviving Voltron, Zarkon sends Yurak back to Arus with a Robeast to finish off what's left of the planet. In the meantime, Pidge goes out on his own to scout the area.
The others venture down deep into the castle to the tomb of Allura's deceased father, Alfor. There, his ghost appears to them and briefly gives counsel. The keys to the lions that comprise Voltron are in his tomb. A problem the explorers face is that they only have 4 of the five keys needed to activate the lions; what's more is the missing key belongs to the main lion, the black one.
Armed with at least some of the keys, they explores are anxious to get in the lions and go help Pidge. Coran hesitates to show them the way to the Lions, but with the Princess' urging, he allows it. As the lions rush off to help, Pidge has already been aided by the Arusians hiding in the caves. They show him the way back to the castle via an underground tunnel.
The lions battle Yurak's forces and do well - until the Robeast is unleashed. The beast swats them away like flies, and the entire force is knocked out from the blows. Princess and Coran, watching from the castle, fear the worst...