Voltron: Defender of the Universe

One of Zarkon's captains is imprisoned after he takes pity on some slaves. Now he's a slave alongside those he once oppressed. As they work on an alien planet, he is saved by a race of natives who live underground. When the Voltron Force arrives, they decide to sneak in and free the slaves. But that's impossible without someone who knows the complex. Can they convince the former captain to help?