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, go to her rescue. But the villain knocks them out with volcanic gas.
To guarantee Betty's safety, Hulk is forced to obey Tyrannus. 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 is forced to do labors for Tyrannus. Fortunately, Jones finds an opportunity to free Betty; untied, Hulk confronts Tyrannus, and beats him.
As the entrance to the underground city is buried, Hulk, Rick and Betty escape to the surface.
Strangely, the traumatic experience makes Betty forget the whole thing.