Zarkon is angered that both Yurak and Haggar have failed in their attempts to bring back the escaped space explorers from Planet Arus. Haggar reassures Zarkon, that her robeast will be able to destroy the Voltron Force before they are able to find the missing key to the Black Lion.

Pidge, slowly makes his way back to the Castle of Lions, where Coran and Princess Allura are worried that the five young men have just risked their lives in a failed attempt to bring back the legendary defender. On his return, Pidge makes his way back to the control room where Coran delivers the bad news.

However, hope returns when Keith, Hunk, Lance and Sven recover from their ordeal and return to the castle. Still, the key to the Black Lion is still missing, so they make one last attempt to find it. Upon returning to King Alfor's tomb, they discover that there was a small mouse hole in the stone coffin. Pidge suggests that they capture the space mice he ran into earlier, in hopes of discovering the location of the missing key.

They capture the four mice, and decide to question them about the whereabouts of the key they took. Allura however, interrupts them, asking them to let her friends go, revealing that the mice were the only creatures that gave her comfort in her years of being alone. On their suggestion, Allura questions her friends and learn that they took the key because they thought it was pretty. Making a deal, they release the mice, and they return with the key to the Black Lion.

Just in time, the Voltron Force then launch to confront Haggar's robeast. The robeast puts up a great fight, and prevents them from forming Voltron, but on their second attempt, they are able to break free from the robeast attack and form the legendary defender. By forming the Blazing Sword, they are able to destroy the robeast.