Jean receives a call from Ted Roberts, warning her that his brother Ralph had been captured and that she must call the X-Men to help him. Jean thinks he knows about her secret identity, but still calls the others and tells the story. Angel and Iceman get the plane, catch Marvel Girl and go find Ralph. Ted tells them that a man called Tyrannus hijacked his brother because his work with cobalt and take him to the core of the Earth. The X-Men then use Ralph's new vehicle, made of cobalt, and go after Tyranus. When they reach a Subterranea, they find a strange guy, but it's not Tyrannus, but Mole Man, his archenemy, who attack the X-Men. Meanwhile, Ralph is finishing his work with Tyrannus, constructing a giant cobalt robot to fight against the giant diamond android made by the Mole Man. After Cyclops and Beast had noticed the missing of his fellows, they follow them into the caves with the help of Cerebro. Later, the two giant androids meet and start a battle, wich ends with the Cobalt one destroyed with the help of Angel, who is aiding the Mole Man because he had been persuaded by Mole Man that Tyrannus was the bad one. Also, Tyrannus discovered that Ralph Roberts had sabotaged his robot while constructing it, because of his evil plans, and when Tyrannus was about to kill him, Angel, Iceman and Marvel Girl saved him. Cyclops and Beast also arrive and attack Mole Man and Angel try to stop them because he is still under mental command of Mole Man. Then Cyclops use an antidote ray on him and finally the X-Men defeat Tyrannus and Mole Man and leave them both with no memory inside one big underground cave.