Following his battle with the Cobalt Man, the Hulk has reverted back into Bruce Banner, just as the ship he was aboard begins taking on water and is sinking. Banner begins to panic as the ship fills with water. But then, the Cobalt Man smashes through the ship to rescue him because he considers Banner to be a victim of radiation like him and pulls him to safety. Taking Banner to the life raft on the surface, his brother Ted tries to talk sense in him. Rejecting this Ralph leaves Banner and swims off to teach the world a personal lesson in the dangers of nuclear weapons.
They are picked up by a US military helicopter containing General Ross, Colonel Armbruster and Betty Talbot aboard. Ross is told by Ted about the danger of his brother, but Armbruster dismisses it as a problem of the Australian government, whom has to be nearby, and they fly off to Sidney to land. While out at sea, the Cobalt Man realizes the full potential of his powers and uses his new atomic energy blasts to carry himself into the air, allowing him to reach his intended destination of Sydney, Australia even faster.
There, Bruce Banner awakens to see Betty sitting by his bed side. The sight of Banner alive and speaking her name upsets Betty enough to make her run out of the room in tears. As Banner begins freaking out about the threat of the Cobalt Man, the so0ldiers in the room try to restrain him while Ross calls out to get tranquilizers to keep Banner under control.
At this time the Cobalt Man arrives and begins laying waste to the city of Sidney with the intent of building up his energies until he reaches critical mass, self destructing and destroying the entire city with him. As he rampages through the city the Australian authorities ask Ross and Armbruster to help fight him off as part of the mutual defense agreement between Australia and America.
Meanwhile, Ted Roberts goes to Betty Talbot and asks her to help him get Bruce Banner to turn into the Hulk so that he can battle the Cobalt Man and stop him. While Ted keeps the guard outside Banner's room distracted, Betty enters the room and finds that Bruce has been drugged up. Finding pills that will counteract the tranquilizers that Banner has been given, she leads Banner to the roof. As they make their escape Cobalt Man continues his rampage, destroying everything in his path, when news cameras arrive he announces to a listening world that he intends to show them the horrors and insanity of nuclear devastation.
While up on the roof of the hospital, Betty decides the only way to get Bruce to change into the Hulk is to try and push him off the building. He stops her before she can, but he is so doped up he stumbles off the ledge on his own and as he falls transforms into the Hulk, saving himself from a fatal fall. Remembering that he has a score to settle with Cobalt Man, the Hulk tracks him down and the two begin to fight across the city. Ultimately, Cobalt Man attempts to kill the Hulk by carrying him up into the atmosphere in an attempt to suffocate him in the vacuum of space. This doesn't work and only works to work Cobalt Man to critical mass, making him explode high in the atmosphere causing no destruction, seemingly killing them both.
Back in the United States, President Nixon and Vice President Kissinger having received the report on the Cobalt Man scenario, decide that they should get into development of a cobalt bomb as soon as possible. While high in the air, the Hulk, having survived the blast is hurtled across the planet falling back into the Earth's atmosphere.