On Doom, Lotor has enlisted slaves to build his super power base with which he hopes to overthrow his father, King Zarkon. Work is over when a Robeast bursts on the scene and destroys the building. It seems Zarkon was wise to the plans and ordered Haggar to do the dirty work, though she hints that she would not have done so if Lotor had just included her in the plan from the start.

Zarkon is annoyed at all the happiness on Arus, so in the dead of night, he sends out a Robeast to start destroying Arusian cities. The Castle's alert sirens go off and it isn't long before the VF launch into action. This crab-like Robeast is a real challenge. What's worse is they're hesitant to use Voltron as the monster insists on staying in the city where frightened Arusians could be hurt by the resulting battle. The VF try to lure the Robeast away but he won't budge, and Pidge only succeeds in getting his Lion skewered by the Robeast.

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?

A ship of young pilots from Galaxy Garrison crash lands on one of Zarkon's slave planets. All are enslaved except for Lt. Lorn, who is saved by a tribal girl with a grudge against Zarkon and a mysterious Fish Man. When Voltron arrives to free the young pilots, a Robeast attacks. The Fish Man is revealed to be one of Haggar's failed experiments. He grows to Robeast size and its' a Robeast versus Robeast battle to save the planet!

On the way home from their latest mission, Hunk spots a planet that is home to some friends of his. He had met these kids on one of his first missions right out of the Space Explorer Academy. Hunk decides to take a vacation and visit them. Haggar and Cossack see a golden opportunity to isolate the Yellow Lion. They arrive on the planet and take the kids hostage. Cossack gives Hunk and ultimatum: Hand over the key to the Yellow Lion, or the kids have had it!

A spaceship crash lands on Planet Pollux appearing to be filled with slaves trying to escape from Planet Doom. Sven and Bandor take them back to the castle for medical treatment. Romelle encounters one patient wearing a mask. She tries to get him to remove it, but he refuses. Bandor tells Sven that his house is on fire. Sven rushes home to find Witch Haggar waiting for him. He also discovers that the masked man and the other slaves were actually sent by Prince Lotor, and have taken the castle hostage. Haggar plans to use Sven to set a trap for the Voltron Force.

The Voltron Force meet Mary Ann, who Coran said was the greatest robotics scientist in the universe. Zarkon's forces have taken one of her robots and turned it into a weapon for evil. Mary Ann, who is a pacifist, gets into an argument with the team over whether or not violence is ever the answer. As she heads for her home planet, Caspia, Keith and the others fear she may be walking into trouble, and decide to follow her. Their suspicions turn out to be right as Haggar and Cossack are waiting. Pidge finds Mary Ann first and provides her with cover fire so she can get away. Haggar then captures Pidge and wants to exchange him for Mary Ann and her robotics knowledge.

The Galaxy Alliance contacts the Voltron Force when it learns that one of its members, Planet Oron, intends to leave. Oron has a new leader, General Kruger, who has built up a stockpile of weapons and therefore doesn't see the need for the Alliance or Voltron. The Voltron Force arrives on Oron to meet with Senator Roland, who wants his planet to stay in the Alliance. Kruger has ordered Roland's arrest, and announces that he has formed an alliance with Planet Doom. After a brief skirmish, Kruger orders the Voltron Force to leave. As soon as they're gone, Cossack takes over Oron, and makes everyone slaves. Roland and some rebels sneak into a radio room, and beg Voltron to please come back.

The evil Rogue Galaxy barbarians have started an all out war against the Alliance. Princess Allura, Keith, Sven, and the rest come to speak at Galaxy Garrison's intergalactic council. But then Lotor crashes the conference, and Allura surrenders herself to save the others. Now the team has to move into action, but who will pilot the Blue Lion?

Zarkon enlists the aide of Stride, a legendary Tiger fighter, in his battle against Voltron. Stride wants no part of Zarkon's personal grudges, so Zarkon falsely claims that Planet Arus has challenged Stride's honor. As Stride leaves to fight Voltron, Zarkon welcomes a new ally, Queen Merla. After hearing of Zarkon's plan, Merla decides to head to the battle sight, because she doesn't want Zarkon getting all the credit. Stride meets Keith in a one-on-one fight. Keith earns Stride's respect by fighting valiantly, which leads to Merla attacking both of them.

Merla uses her telepathic powers to make Zarkon place her in charge of the next mission to attack Planet Arus. Meanwhile on Arus, word has been spreading that King Alfor is preparing to return from the grave. Some townspeople find a cave that has a statue with a golden shield that is believed to have belonged to Alfor. Allura, wanting to believe that her father has returned, heads for the cave with her teammates right behind her. It turns out to be a trap. The guys get trapped in the cave, and Allura gets taken prisoner by Merla.

Zarkon oversees the wedding of Lotor and Merla. He sees the wedding as the way to unite their two evil kingdoms. Meanwhile on Planet Arus, a young woman named Taryn pines away for her lost love, Ro, who was taken as a slave to Planet Doom. Ro shows up with two mystery women, Merla and Haggar in disguise. Merla claims to have a crush on Keith, and in exchange for bringing Ro home, she wants Taryn to deliver a jar of fresh honey to the castle as a present for his birthday. Taryn gives the jar to Allura, who ponders her own feelings for Keith. After Taryn returns, Ro reveals that the jar actually contains poison. Now they have to get to the castle and warn the Voltron Force before any of them drink it.

Coran is showing the Voltron Force space images of the planet Philos - an oddity in that it's a planet that orbits and acts like a comet. Ineterrupting their space lesson is a skull tank, which lands on Arus with a white flag raised. Though skeptical, the team has to check it out. Alliance guards escort the pilot of the tank out, which is an alien who claims to be from Philos. Lance wants to just lock him up right away, but Allura gives the stranger a chance.

To cure a sick townswoman, Allura searches for a flower that only blooms once every one thousand years. Haggar steals it to load Voltron into a trap, but Sven takes things into his own hands before Keith and the team arrive. He rounds up his band of freedom fighters and steals a skull tank, attacking Zarkon himself. Meanwhile, Lotor hatches a new plan to bring down his father with Merla's help.

The Muses of Norn escape from Merla and reach Galaxy Garrison safely. Not only that, but they've managed to bring Merla herself as a prisoner. They'll only turn her over to a high ranking official, so Prince Bandor goes. Then Queen Merla springs her trap with the help of Lotor. The Voltron Force must rescue Bandor while fighting two evil starfleets at once.

All at once, Zarkon's countless slave planets begin revolting at the same time. The evil King's armies are spread thin, and the Alliance stages a major offensive. Princess Allura and the Voltron Force once again take the fight to Zarkon. Zarkon fights back by trying to sacrifice Prince Lotor, but Merla has a change of heart and protects him. Then she begs the Voltron Force to help her save Lotor from the Pit of Skulls.

Voltron continues the attack on Planet Doom with Merla's help. This time, Lotor is by her side, ready to take on his father and lead Voltron though Haggar's traps.