While walking down a railroad track, the Hulk confuses an oncoming train for an attacker and punches it off the rails. He furiously begins ripping the train apart demanding to know why humans can't leave him alone. When no answer is forthcoming he flees the scene, ending up landing in Lake Michigan. Swimming across the lake to the city of Chicago, the Hulk pulls himself into an alleyway to rest, and reverts back into Bruce Banner. Banner awakens and realizes that he has no money left and needs to get a job.
Finding a newspaper ad looking for a janitor at Soul-Star Research Ltd, Banner finds some clothes and goes seeking the job. He gets it right away and begins cleaning up around the science lab. This lab happens to be run by Stan Landers, Mark Revel and Alexandra Knox. Both men love Knox, but she loves Stan alone. They are currently working on a device that will hopefully absorb electrical brainwaves released into the atmosphere and theoretically gain access to every thought throughout human history. As they are working on their first test of their device, Banner enters the room to clean up. Overhearing them having troubles he offers them his scientific expertise, and when his advice works they are surprised to find that they world famous Bruce Banner is working as a meager floor moper.
Inviting Bruce to assist them, they all put on special suits and activate the electronic cannon that would absorb the electrical brainwaves. However, instead of pulling the thoughts of humanity over the ages they bring back something far more deadly: The energy creature known as Zzzax. The resulting explosion from Zzzax's resurrection on Earth knocks Mark out send Stan to his rescue. Although Marks pulled to safety, Stan is consumed by Zzzax, his dying thoughts are his love of Alexandra. As Alexandra and Mark mourn the death of Stan, Banner suddenly transforms into the Hulk. Zzzax meanwhile has not only absorbed Stan's intelligence, but also his love of Alexandra, and after knocking the Hulk out, grabs the girl and smashes out of the building and into the city of Chicago.
Meanwhile, at Hulkbuster Base in New Mexico, Colonel Armbruster continues his investigation of Glenn Talbot. Finding something suspicious about his return from Russia, Armbruster has one of his men send in a cellu-scanner device to scan his sleeping body.
While back in Chicago, Zzzax has taken Alexandra up to the top of John Handcock Center to keep people away from him and Alexandra. As a heavy rain begins to fall the Hulk chases after Zzzax and tracks him down to the building. Swatting aside the police the Hulk attacks Zzax, and during the fight Mark arrives in a helicopter. As the Hulk battles on, Mark manages to fire a wire through Zzzax and flies his chopper up into the sky where it is struck by a bolt of lightning. The lightning streaks down the cable and causes Zzzax to discorporate once more. Seeing Mark alive, but falling to his death, the Hulk saves his life and leaves him with Alexandra as the storm clears and dawn begins to break. As Mark and Alexandra hold each other, the Hulk walks off to face a new day.