The Voltron force are out in their Lions doing target practice (this ep contains the part where, when Pidge does a less than great job in practice, Keith lectures him. To which Pidge notes "Did you know your nose wiggles when you're upset," to which Keith retorts, "My fist wiggles too!"). The team becomes alarmed when their gauges go nuts harkening the presence of a major power source in the area. They rush back to the Castle to see what Coran knows of the disturbance. It's not long before they realize the problem lies in a Robeast that has the ability to put out a force field which not only blocks radar, but disables their power sources. They even fear the Lions would be too affected to be of any use.
Keith's plan is to go at the Robeast under water before it can get to the castle, but since the Castle is out of power he has to use an old underwater sled device to get there. The rest of the team have to try to repair the remaining sleds so they can join Keith. Keith makes it to the robeast and tries to attach some mines to it. As Keith swims away before the blast goes off, Lotor's clone - who had been surveying the scene from atop the robeast - notices Keith's silouhette in the water. Keith and the clone end up facing off. The real Lotor, watching from the safety of his ship, is anxious about the clone's performance as the battle is being broadcast all over doom. Defeat for the clone would mean utter humiliation for him.
Keith is suspicious about Lotor's sudden fighting prowess and almost is killed when the mines finally detonate. The blast knocks the clone back which gives Keith a second chance. The blast also interrupted the beasts' electronic hold on the Castle which allows Coran a chance to survey the situation and locate what he believes is the main power source for the Robeast. While Coran is hatching a plan, Lotor 'mind merges' with is clone, ordering it to finish Keith off so he can look good. The clone rebels however, digusted as he is with the real Lotor's cowardice (I love this line of the real Lotor's: "You are myself, and you're making a fool of me.") During their exchange, Keith figures out the Lotor before him is a clone.
The Robeast is reactivated when the clone turns on the real Lotor and as a traitor, it is promptly destroyed by the beast. The robeast then turns its attention on Keith while the others launch in their Lions to assist. Lance, in the one water sled they managed to repair earlier, gets there first and saves Keith. While Keith and Lance speed back to the Castle on the sled, the rest of the team fight the Robeast as best they can. Of course, once Keith and Lance return Voltron is formed. The lion head attack coupled with the blazing sword makes short work of the Robeast.