Following the Thundercats' rescue by Panthro, Lion-O attempts to start a conversation with him as he makes repairs to the Thundertank. However, Panthro constantly attempts to avoid talking showing little respect for Lion-O's authority. Eventually, Panthro says he needs to get Thundrillium to recharge the Thundertank from a nearby Lizard-controlled mine run by Grune with the aid of the Driller. Initially, Panthro plans on doing it alone, but Lion-O, Tygra and Cheetara volunteer to come along. In response, he tells them not to slow him down. While searching for the Thundrillium, Panthro has flashbacks of his friendship with Grune ranging from meeting on the battlefield and saving Claudus from attacking Lizards, rising through the ranks as officers, being sent to recover the Book of Omens, and Grune's eventual corruption and betrayal to Mumm-Ra, unleashing the villain from his imprisonment. Panthro and Grune eventually meet up in the mine. But before they can have a showdown, the Driller appears to block the Thundercats long enough for Grune to escape. In the course of the battle with Driller, Lion-O uses the Sword of Omens to save Panthro's life and destroy the robot, but all its digging causes the mountain to collapse, with Cheetara only barely getting the Thundrillium they came for. After escaping the mine, Panthro says Lion-O has earned his undying loyalty. But when Lion-O asks if he can drive the Thundertank, Panthro says 'Not a chance... my king'.