Dr. Brundle (Jeff Goldblum) invents a teleporting machine, then tries it on himself. Unfortunately a fly enters the transportation chamber with him and in the process their genes got mixed.
Brundle's body and mind start to change -- he is a lot more powerful than before, he can think better and faster, but then he begins to transform into something he later name 'Brundlefly' -- he loses nails, then teeth,
His girlfriend (Geena Davies) is pregnant but she doesn't know whether it's a Brundle or a Brundlefly baby, so she wants an abortion. The Brundlefly kidnaps her right from the operation table.