Pogue's draft: Geoff and Barbara Powell are a happily married couple. Geoff, a brilliant scientist, has been working on a teleportation machine, but is unwilling to tell his employer, Phillip DeWitt, or his friend, Harry Chandler, about the nature of the project.
DeWitt is greatly displeased by this, and threatens to pull his funding of the mystery project unless he is given full disclosure. After several failed experiments, such as a monkey's atoms never reintegrating after disintegration, Geoff eventually is successful in teleporting both inanimate and living objects.
However, when he tries it on himself, a housefly slips into the booth with him. Seemingly normal at first, Geoff soon develops incredible strength, stamina, and energy.
After sprouting fly-hairs and losing his fingernails, Geoff eventually discovers that the fly has been absorbed into his body, and that its cells are now taking over his own.
As he slowly mutates into a giant fly, Geoff loses body parts, and becomes able to climb walls and digest food with corrosive vomit. Barb is horrified to learn that she is pregnant by Geoff, and cannot be sure if the child was conceived before or after his teleportation.
Eventually, Chandler discovers the teleporter's existence, reveals it to DeWitt, and demonstrates it on a cat, only to have the lost monkey atoms return from the ether and create a horrible "monkey-cat" creature, which DeWitt beats to death with a metal rod.
Despite this failed experiment, DeWitt sees the substantial monetary value of the device, and so takes possession of the teleporter. Geoff (now mostly transformed into a fly-monster and unable to speak) learns of this, and goes to DeWitt's office building, followed by Barb.
Geoff confronts DeWitt, starts a fire in the lab where the teleporter is now housed, and kills DeWitt by vomiting and feeding on him. He then traps himself in one of the teleportation booths just as Barb arrives to watch the fire kill him and destroy the teleporter—his intent all along.
In a coda, Barb dreams of giving birth to a giant maggot, only to wake up in a hospital, where it is revealed that she's given birth to a healthy baby boy.
Cronenberg's draft: The revised script differed greatly from Pogue's screenplay, though it still retained the basic plot and the central concept of a gradual mutation.
Cronenberg rewrote the characters and most of the dialogue from scratch (as well as fusing DeWitt and Chandler--who had romantic intentions toward Barb in the Pogue draft--into Stathis Borans), and carried over a few key moments and concepts.
Certain aspects of the transformation from Pogue's draft (such as the hero's loss of body parts) were expanded upon, and Cronenberg also layered in his trademark themes of sexuality, body horror, and personal identity.
He also made it a point to keep Seth Brundle as articulate as possible for as long as possible, as opposed to Pogue's draft, in which Geoff Powell loses his ability to speak two-thirds of the way through the script.
Seth Brundle's increasing mania and personality changes in the early stages of the transformation were emphasized in the rewrites, and the notion of the transformation itself being a horrible (and very metaphorical) disease became a key factor in the new script.
Also, in this version, Brundle was clearly transforming into a bizarre hybrid creature as the result of a genetic fusion.
Whereas in Pogue's version, Powell was being taken over by the fly's cells, which had been absorbed into his body (thus slowly transforming him literally into a giant fly, rather than Brundle's deformed man-fly mixture).
Cronenberg's version also retained such moments as Brundle catching a fly in mid-air, the fingernail-pulling, and the maggot-baby dream (which was moved to an earlier point in the story, and used for thematic and plot purposes rather than as an end-of-film shock moment).
The "monkey-cat" of Pogue's script was repurposed by Cronenberg into a twisted, desperate attempt of Brundle's to find a cure, and Pogue's sequence of a fly leg hatching from Geoff's side was taken one step further, with Brundle amputating the twitching limb with his teeth.
Pogue's script also featured a baglady being murdered by Geoff in an alley, and Cronenberg revised this so that the woman was killed by vomit-drop (as with DeWitt's murder at the end of the original draft) rather than Geoff cutting her throat accidentally.
However, Cronenberg never filmed his version of this sequence, which was written out of the final shooting script.
While Cronenberg's script did not end with Veronica Quaife giving birth, it did end with a coda which revealed that she was now pregnant with a normal baby, conceived by Borans after Brundle's death (and the abortion of Brundle's possibly tainted fetus).
Despite the extensive rewrite of Pogue's script, Cronenberg insisted during Writers Guild arbitrations that he and Pogue share screenplay credit, since he felt that his version could not have come to pass without Pogue's script to serve as a foundation.
With a script that everyone was now happy with, Cronenberg assembled his usual crew and began the process of casting the picture, ultimately deciding on Jeff Goldblum and Geena Davis for the leads.