Seth Brundle is working on a project that will change the world: a set of "Telepods" that allows instantaneous teleportation of an object from one pod to another.
Brundle decides to teleport himself, but a common housefly slips into the pod, unseen. The teleportation successful, Brundle emerges from the receiving pod, seemingly normal. Shortly after his teleportation, Seth begins to exhibit what at first appear to be beneficial effects of the processósuch as increased strength, stamina and sexual potency. However, he soon becomes arrogant and violent, and eventually realizes that something went horribly wrong when his fingernails begin falling off. Brundle checks his computer's records, and discovers that the telepod computer, confused by the presence of two separate life-forms in the sending pod, merged him with the fly at the molecular-genetic level.
He has started to exhibit fly-like characteristics, such as vomiting digestive enzymes onto his food in order to dissolve it, and the ability to cling to walls and ceilings. Brundle comes to realize that he is losing his human reason and compassion, and that he is now being driven by primitive impulses he cannot control.