Dr. Neil Hamilton was a biochemist employed at Hamelin University, where he worked under a government grant to study plants in order to finds a means of solving the world's hunger problem. Upon receiving a phone call from his old mentor Dr. Kate Travis, Hamilton went to her private clinic where she showed him Bobby, a patient of hers grown large due to an experimental growth formula Travis had been developing. Travis, unable to find a means of reversing the chemical's effects, hoped that Hamilton, he best student, would be able to.
Hamilton took a sample of the formula back with him to Hamelin, which was under fire from a group of animal rights activists led by Mark Hales, one of whom, Alex Reed, Dr. Hamilton was romantically involved with. Although initially Hamilton was reluctant to use animals in his work, after he and his assistant Joshua made a tomato plant grow large as a test, Hamilton was convinced that they would eventually need to progress to animal testing phases if they wished to cure Bobby. Thus Hamilton injected a single lab rat with the growth formula.
However, Mark and his fellow activists broke into the lab and were attacked by the rat, which had grown gigantic overnight. Mark was killed and the giant rat escaped, whereas the other rats got loose and ate the giant tomato plant, thus eventually becoming large themselves. The rats infested the Hamelin campus, killing several people, including Joshua, and even though Hamilton tried to warn Dean White of the danger, White was more concerned with not postponing the university's upcoming swimming competition. Hamilton's repeated efforts to persuade Lieutenant Weizel of the police to shut the campus down caused White to fire him.
However Hamilton broke back into the university and was able to complete the antidote to the growth formula. After his rival scientist Professor Edmond Delhurst accidentally died of a "super cancer" due to fooling around with cancer cells and the growth chemical, Hamilton tested the antidote on Delhurst's tumorous corpse, halting the growth of the cancer and proving that the cure worked. Afterward, he and Alex assisted Lieutenant Weizel in dealing with the more immediate problem of the giant rats, which attacked the swimming competition, killing even more people, including Dean White.
Since Delhurst had tested the growth formula out on Hamilton's pet rat Louise, Hamilton decided to use Louise to lure the other giant rats (who were all males) into the campus courtyard where the police could gun them down. To Hamilton's dismay, Louise was not able to get out of the line of fire in time as killed in the crossfire. Once Weizel and his men had killed all of the rats, Hamilton went to phone Dr. Travis about the successful antidote, only to discover he was too late and that the giant, now-psychotic Bobby had killed Travis and escaped.