The Missing Key
Pidge finally makes it back to the castle, briefly encountering the Space Mice. After passing out briefly from exhaustion, he's upset to learn the fate of his friends. That is, until he realizes he can hear Hunk snoring over the intercom, which tells him the team is still alive. The team awakes and flies back to the castle.
Now, their focus is on finding the missing key before Zarkon's forces have a chance to recharge the Robeast and attack again. They suspect the Mice and proceed to trap them, which upsets the Princess. It seems the rodents have been her longtime friends. Luckily, they willingly tell her they have the key and go retrieve it for her. It's just in time, too, as Zarkon's forces move in for the kill.
All five lions launch and for whatever reason, try to take on the Robeast without Voltron. They do have the good sense to form Voltron, which makes short work of the Robeast.