Synopsis:
Scientist Sebastian Caine (Kevin Bacon) has developed a serum that can make a subject invisible. Working late one night he restores a test gorilla to visibility and immediately reports his success to the top members of his staff.
Sebastian becomes obsessed with his ex-girlfriend Dr. Linda McKay (Elisabeth Shue). Unbeknownst to him, she has become involved with Dr. Matt Kensington (Josh Brolin). Sebastian and veterinarians Sarah Kennedy (Kim Dickens) and Carter Abbey (Greg Grunberg), and technicians Frank Chase (Joey Slotnick) and Janice Walton (Mary Randle) – eventually enable the serum to work on the gorilla. To Sebastian, it feels like the end, as reversion was the last big breakthrough.
Instead of reporting his success to the military, Sebastian inexplicably lies to the oversight committee, including his mentor Dr. Howard Kramer (William Devane), and talks Linda and Matt into going right into human testing, the subject being Sebastian himself. The procedure is successful. He conducts some clinical tests on himself, and molests Sarah in her sleep. Three days later, the reversion fails.
The researchers construct a latex mask for Sebastian to wear around the lab. Matt discovers a promising way to make Sebastian visible again. A computer simulation, however, fails. Sebastian heads back to his apartment for things to take back to the lab. There, he spies on his neighbor (Rhona Mitra) and rapes her while fully invisible.
Linda warns him that if he leaves again, she and Matt will tell Dr. Kramer what they did. Linda has a dream of being raped by an invisible man. Ignoring their threat, Sebastian assembles a device that runs a video loop of his heat signature in his quarters; Sebastian is out spying on Linda and Matt, becoming enraged when he sees them having sex.
Seeing Matt and Linda together, Sebastian smashes their bedroom window. The two call Frank, who tells them that Sebastian is still in bed. Meanwhile, an enraged Sebastian returns to the lab. Unable to stand the barking of an invisible dog, Sebastian kills it by brutally smashing it against the cage wall. The team soon discover that they have been watching a recording and that Sebastian has been getting out without their knowledge. Linda and Matt go to Dr. Kramer's house and confess their experiments. The disappointed Kramer fires them. After they leave, Kramer attempts to warn the military, but Sebastian, who followed Linda and Matt to the house, drowns him in his own swimming pool.
The next day, Sebastian waits until all of the team is in the lab and then disables the phones and all of the elevator codes except for his own. He removes his clothing and latex mask and, invisible, begins to hunt them all down. Janice falls behind while the team searches for Sebastian and he strangles her with an IV line.
Linda, Matt, Sarah, Carter, and Frank learn what Sebastian is up to, realizing that they are the only ones who know what he has become, and therefore he is planning to kill them all. Linda, Sarah, and Frank hide in the lab while Matt and Carter take tranquilizer guns to hunt for Sebastian using thermographic imaging goggles. Matt shoots at a heat signature that turns out to be from a steam vent. When Matt turns around, Carter has been yanked into the air by Sebastian, who is on top of a pipe. Matt tries to shoot him, but Sebastian throws Carter down toward a steel bar, which hits his carotid artery, causing massive blood loss. Sebastian almost kills Matt, but Linda drags him to safety.
Sarah and Frank find the barely conscious and severely injured Carter. Despite Frank's warnings, Sarah runs to the freezer to get blood for a transfusion, and is killed by Sebastian. After Carter has bled to death, Linda, Matt, and Frank go after Sarah, finding her dead in the walk-in freezer. Frank sprays a fire extinguisher to expose Sebastian, but Sebastian comes from behind and impales Frank with a crowbar. He then stabs Matt with the hook end, locks Linda and Matt in the freezer, and leaves them to freeze to death.
Sebastian puts on his latex mask and clothing, spending time to ensure he looks near-human. Meanwhile, Linda covers up Matt's wound with duct tape. She constructs an electromagnet using a defibrillator and other lab equipment, and uses it to open the freezer door. She lights a fire to warm Matt and gathers the parts needed for a flamethrower. Sebastian goes to the lab and creates nitroglycerin to blow everything up. He puts the nitroglycerin in a centrifuge, starts a timer and destroys the keyboard, so it can't be stopped. Just as he is entering the elevator to leave, Linda appears and fires a crude blowtorch at him. His latex and clothes burning away, Sebastian barely manages to escape the flames. Linda uses the flamethrower to start the ceiling sprinklers, partially revealing him. The two fight; just as she is about to lose, Matt appears and hits Sebastian with the crowbar. Sebastian recovers and approaches Matt and Linda from behind with the crowbar. Just as he swings it down on them, Matt turns and deflects the blow, throwing a wet Sebastian into a nearby circuit box, and he is electrocuted. As the electricity courses through his body, Sebastian is rendered partially visible: his muscles, bones, and organs can be seen, but his skin and what's left of his hair remain invisible.
Linda and Matt find the nitroglycerin about to explode, and decide to abandon the lab, climbing up the elevator shaft. The lab explodes filling the shaft with fire. The two are almost out when Sebastian appears. He fights with Linda, and kisses her "for old time's sake" before she grabs the elevator cable and knocks the car loose, sending Sebastian falling to his death into the inferno below. Linda and Matt emerge from the burning laboratory, and medics take them away in an ambulance.
