The Voltron force launches for Doom, but is attacked by an invisible force while in space. Zarkon appears on the Lions' telecoms to warn them to turn back. The team counters with the proposal that they will if Zarkon frees his slaves. Of course, he refuses. Voltron is formed and he's ready to kick butt. First, they run into Lotor and don't take him seriously when he says he wants to help. Instead, they go against his advice and fly right into Haggar's trap. Now in a jam, the team listen to Lotor's advice about how to get out of the situation and it works. Their faith in Lotor is much less shaky now.
Hunk is still pretty skeptical, and Lotor does his best to explain how his turning to good came about. Then Merla shows up spouting similar sentiments while also explaining how her and Lotor are going to get married. It's decided to join forces to stop Zarkon once and for all. As soon as they get to Doom, Haggar has a robeast waiting. Thanks to Lotor and Merla's help, Voltron finishes off the beast and it's on to deal with skull tanks. Those too are easily defeated.
The next trick is to use threats against Arusian slaves to get Voltron to surrender. It almost works until Lotor reveals they aren't slaves, but merely robots. From there, it's on to demolish Castle Doom. The VF get out of Volton to survey the damage when Lotor has to save them yet again from snipers. Lotor leads them inside the rubble to capture Zarkon. Before going into Zarkon's throne room, the Princess remarks about how it's nice to see Lotor acting like a real Prince. He proposes to her on the spot, but she rejects him by reminding him that he's pledged to Merla.
That bit of awkwardness out of the way, the team goes into the throne room. Zarkon is there, but he quickly traps the team within a glass cage and releases a tentacle bomb inside. But Lotor uses the moment to reveal that Zarkon is really Cossack in disguise and that he himself has not gone to the good side afterall. It was all a trick. Pidge, luckily, has a device in his belt that blasts a hole in the glass case and allows them to escape.
In the meantime, Merla confronts Lotor and is disgusted that he wasn't serious about turning good. The blast from the tentacle bomb causes Doom to start to fall apart, and the teams must race to their Lions. But first, Keith has to confront Lotor one more time. He manages to escape in Black Lion before the planet goes.
That's the end of Zarkon and Doom, right? Wrong. Zarkon reveals at the end that they were lead to a false planet Doom with a false castle. The joke's on Voltron this time.