This film is a remake of Kurt Neumann's The Fly (1958). This film directed by David Cronenberg has the same context and story as the original but the screenplay and continuity differ greatly. Cronenberg does a great job of modernizing the film with the body horror and the way the couple come together through a Science venue. Cronenberg has made the transformation a cancerous change which some people would call a metaphor for HIV which was present in the minds of the public at that point in time.
As Veronica Quaife (Geena Davis) starts to form feelings for Seth, she wants him to succeed and try to get his teleportation device completely functional, and supports him through his trials and successes. After a hideous error involving a baboon in the device this upsets Seth but veronica still supports him and wants him to succeed. When Veronica receives a letter in the post form her boss and ex partner Stathis Borans (John Getz) she wants to go and finish it once and for all. Seth in his paranoid, delusional state thinks that veronica is getting back with her partner and recklessly gets in the teleporter and teleports himself.
After Seth is feeling reborn and alive as you see him doing acrobatic feats. After he has had sex with veronica when they both come to realise he got into the teleporter with a fly, and she finds out she's pregnant she wants an abortion. Now at the time this was a very new and challenging view. This film has really tested to the limits what and when it should be acceptable to have an abortion as it is likely the child be likely be a Human-Fly hybrid.
As Seth now known as Brundle-Fly accepting his inevitable fate of turning into the monster. He is angry at first at the mistake he has made and tries to reverse his situation but to no avail. As the cancerous change develops and rather inevitably like HIV he slowly accepts the change, and he becomes more deformed and changing into a monster his comedic personality still shines through reflecting his humanity.
When Seth fully transforms into the fly hybrid he becomes more insect than human and wants the three of them himself, Veronica and the unborn child to be spliced together in the teleporter. After Stathis rescues Veronica, Seth gets teleported with part of the device. He slowly crawls towards veronica holding the gun and holds it to his head, which shows very powerful human emotion. He has come to realise what a monster he has turned into. Both movies have the same general theme showing how powerful human emotions can be even after they have been degenerated into monsters.