David Banner masquerades as David Bellamy, a janitor, to gain access to Genecore Laboratory. He believes that studies of one of the scientists there, Dr. Ronald Pratt, may hold the key to curing his gamma-induced condition that, in times of stress, turns him into a superhuman creature known as the The Hulk. At nights David breaks into Pratt's high-security laboratory and examines his work, making corrections and additions. Ronald notices his alterations and hides in the lab in wait for his secret helper. He catches David, who under threat of the authorities being summoned, reveals his true identity and his transformations into the Hulk. His condition dives into Ronald's own research on using genetic engineering to enhance humans' healing ability, for the Hulk's accelerated metabolism closes any wound in seconds.
With the help of his scientist wife Amy, Ronald constructs a force field cage and sensors which enable them to study the creature. The next few months, David, Amy, and Ronald bond while working on both Ronald's healing project and a cure for David. The facility's board pulls Ronald's funding because he refuses to turn his research toward engineering super soldiers, forcing him to move up David's treatment. An Eastern European spy named Jasmine is ordered by her superior Kasha to steal Ronald's files. Since Jasmine has attempted to leave the espionage business before and believes that Kasha is selling the information they acquire for his own financial benefit, Kasha holds her sister Bella hostage to ensure her compliance. Jasmine infiltrates the lab during David's treatment, expecting no one to be there since the procedure was unauthorized. Ronald tries to stop her from taking his files and she shoves him into a table of chemicals, leaving him in a coma. She pulls him out of the way of the resulting fire, but anger at her disruption and assault has already transformed David into the Hulk. The Hulk attacks Jasmine, who flees, and carries Ronald to safety before escaping the security officers alerted by the smoke alarm.
Since the information in the files Jasmine took is insufficient, Kasha orders her to abduct David in the hope that he can fill the gaps. However, after Jasmine finds David, her co-agents act on orders to eliminate her as unnecessary. During the resulting fight, one of the men is mortally wounded. He tells Jasmine that Bella is the leader of the spy network and faked her own hostage situation. David takes Jasmine to her cabin hideaway and treats her bullet wound. In return, Jasmine uses her espionage skills to get David to Ronald's hospital room without being identified so that he can speak to Ronald, coaxing him out of his coma.
The Hulk saves Jasmine from another attack by Kasha's men, causing her to see David with new eyes, and they become romantically involved. Plans to leave the country in order to elude their pursuers are aborted when they hear the Pratts have been kidnapped by Bella. Jasmine extracts their location, an airfield, from one of her former comrades and tips off the authorities. While sneaking into the airfield hangar Jasmine is caught by the network's torturer, Zed, but she misdirects and overcomes him by pretending to swallow a suicide pill. The police raid the airfield, and David gets the Pratts to safety. Bella boards an airplane with Zed and shoots Kasha, who had been making overtures of taking her place. Disregarding her orders to escape, the vindictive Zed tries to run Jasmine down with the plane. David runs out to protect Jasmine and turns into the Hulk. The Hulk breaks into the plane. Bella opens fire on him, unintentionally shooting the fuel tank. The plane explodes, killing Bella, Zed, and the Hulk. David transforms back to human before succumbing to his injuries. Jasmine, Ronald, and Amy mourn for him.
