Science journalist Veronica "Ronnie" Quaife is sent to an inventors' showcase for a story, where she's especially intrigued by Seth Brundle's claims that he's working on something that will change all of humanity. They go to Seth's apartment where he displays his "telepods" that can transport matter, but Seth is thrown to realize she intends to write a story about them before they're fully functional.
Luckily for Seth, Ronnie's boss Stathis Borans assumes he's a con man, and he's able to convince her to spend more time together so she can write a whole book about the process, explaining that the telepods still can't transport organic matter. He demonstrates with a baboon, which comes out the other side a horrific mess of flesh that dies shortly afterward.
As they spend more time together Seth and Ronnie fall in love, and some pillow talk about the primal desire to be close to flesh gives Seth an idea. He cooks a steak after teleporting half of it, and that half ends up distastefully synthetic, proving that the telepods need to be taught the same "crazy" instinct for organic flesh. Sure enough, another test with a baboon is successful.
Stathis, who also previously dated Ronnie, grows jealous of Seth and starts stalking her, and even sends a magazine cover about the telepods, implying he's going to steal her story. Ronnie runs out to confront him without explanation, leaving Seth thinking she's going back to him. He gets drunk, and ends up deciding to teleport himself, not noticing that a fly that had entered the telepod before him, and was perching on top of one of its windows.
Ronnie comes back the next morning and Seth tells her about the successful teleport, leading to them sleeping together again, but as the day goes on it becomes clear that he has a far more aggressive personality. Ronnie also finds strange hairs growing out of his back, and when she declines to be teleported, Seth flies into a rage and goes out to find someone else. He arm wrestles at a bar and gruesomely breaks his opponent's arm, impressing the man's date Tawney into coming back with him.
Tawney is also hesitant to be teleported and Seth tries to force her through, but she's saved when Ronnie comes back and reveals she had the hairs tested, and found out they're actually from an insect. Seth furiously breaks up with her and throws her out, but then finds his fingernails peeling off. He consults the telepod computer and is horrified to learn about the fly.
A month later, Seth calls Ronnie and begs to see her again. She arrives to see he now appears to be covered with cancerous growths, and he explains that the telepods were confused to find two organisms and fused them together genetically. He's also now forced to eat by vomiting corrosive acid to liquefy his food, and able to stick to walls, and convinces Ronnie to continue documenting his change into a brand new creature.
Ronnie finds that she's pregnant, with no way to tell if it's from before or after Seth teleported. She tries to tell Seth, only for him to ramble about how he's now "an insect who dreamed he was a man," and orders her out of his life as he'll only hurt her. Ronnie gets Stathis to take her to get an abortion, only for Seth to kidnap her as the baby could be all that's left of his original self.
Stathis goes to the lab with a shotgun to rescue Ronnie, only for Seth to ambush him and use his vomit to melt his hand and foot until he passes out from the pain. He then explains his new plan to Ronnie, for them to go through the telepods together so both of them and the baby can be in the same mostly human body. Just then, his transformation finishes in an agonizing display of his final humanity melting off until only a hideous fly creature is left. Ronnie's screams as she's thrown into a telepod wake Stathis up, and he painfully manages to shoot the connecting wire to the computer, removing her from the teleportation. Seth breaks out of his own telepod, but only gets halfway out before the sequence engages and splices him with the telepod itself, leaving him in unimaginable pain with the metal fused throughout his body. In one last burst of humanity, he grabs the shotgun barrel and places it to his own head, and Ronnie regretfully puts him out of his misery before collapsing to the floor in tears.
