Tyrannus, a centuries-old leader of an underground race, has kept his body young with the Fountain of Youth. He intends to conquer the surface world and kidnaps Betty Ross.
Bruce Banner uses his gamma ray machine to turn into the Hulk, and, joined by Rick Jones, goes to her rescue. But the villain knocks them out with volcanic gas.
Tyrannus forces Hulk to obey in order to guarantee Betty’s safety. For the tyrant’s amusement, the monster fights a powerful robot in a Roman-like arena. Hulk beats his opponent, and tries hurting Tyrannus, but the villain knocks him out (again) yet this time using a paralyzing weapon.
Hulk has to do labors for Tyrannus. Fortunately, Jones finds an opportunity to free Betty; untied, Hulk confronts Tyrannus, and beats him.
Hulk, Rick, and Betty escape to the surface because someone buried the entrance to the underground city.
Strangely, the traumatic experience makes Betty forget the whole thing.
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