Kevin Bacon is Sebastian Caine, a brilliant scientist who is the project leader on a secret government funded project to make an invisiblity serum. So far the team has already found a way to make living things invisible, but haven't found a way to bring them back without liquifying them. Working late one night, Caine plays around with a display of a molecule and figures out how to bring the test subjects back. Calling his partner, Linda McKay (Elisabeth Shue) he tells her to get everybody down to the lab and to start the testing. Linda wakes up her boyfriend and project member Matt Kensington (Josh Brolin) and informs him of the news. We then find out that Sebastian and Linda were once lovers, but Sebastian's arrogance and the fact that he has an ego the size of the Rose Bowl got in the way. And Linda's worried that Sebastian will find out and get jealous. Gee, I wonder if this is going to pay off?

At the giant underground lab where the scientists work, Matt prepares to sedate Isabelle, an invisible gorilla for testing. The gorilla bites him though, then runs off down the halls. Matt runs into Sebastian, who helps capture Isabelle using heat sensor goggles and a dartgun. Isabelle is tied down where the other scientists Frank (Joey Slotnick), Janice (Mary Randle), Sarah (Kim Dickens), and Carter (Greg Grunberg) help out. The gorilla is given the serum, (in one of the coolest scenes of the movie) which painfully but successfully brings the gorilla back to a visible state. Sebastian is then given another chance to show us he's an jackass, by joking with Sarah, the veteranarian, about dissecting Isabelle and analyzing her brain.

Afterwards, Sebastian, Linda, and Matt are giving a presentation to the Pentagon, where Sebastian lies and tells them that the reversion serum didn't work and that they will need more time. The head funder of the project, Dr. Kramer (Willaim Devane) tells Sebastian he'd better hurry on the project, threatining replacement. Linda and Matt then get pissed at Sebastian, wondering why he would lie to the Pentagon about the project. He reasons that if they knew about the reversion serum, they would take over and fire the project workers. He then tells them that he wants to move to Phase III: human experimentation. And he wants to be the test subject. At first Linda and Matt are reluctant, but they eventully agree with him, but they don't tell the rest of the team about the renegade testing. He plans to test the invisiblity serum on himself, perform tests on him for three days, and then return him. The team agrees.

The next day, Sebastian is ready to become invisible. Strapping him down at the same table the gorilla was, a very naked Sebastian injects himself. He painfully goes through various stages of invisibility, where his body vanishes layer by layer. Linda then pulls rank and becomes the project leader, which makes Sebastian mad, seeing how he has no more control over the experiment. The first night Sarah is left to watch him, but falls asleep and is fondled by Sebastian. The day after, Sarah, who suspects he did something, but can't prove it. The team does more tests, and Sebastian playfully screws around with them.

After three days, the team prepares to restore Sebastian to normal. They inject the reversion serum into him, but it brings him back to partial visibility, but it fails, reverting back to invisibility. The research team then gets to work on bringing him back, but can't figure out how. They make a latex face mask for him to wear, and perform even more tests on him. Later that night, Sebastian finally gets sick of the isolation, and the fact that he can't leave the lab. He breaks the rules, and leaves for his apartment. He has fun on the outside, scaring a couple of kids in a car next to his.

At his apartment, Sebastian notices his beautiful neighbor (Rhona Mitra, the model for Lara Croft) through his window, getting ready to take a shower. He takes off his mask and clothes and heads for her apartment. At first he messes with her mirror while she's using it, then he attacks her. Although it's unclear what happens, it's pretty much implied that he rapes her.

Meanwhile Linda and Matt find out Sebastian left the compound, and go to his apartment, finding the mask and clothing in a pile on the floor. They head back to the lab, along with the other members of the team. They talk about what to do with him, but are surprised when he comes back. Linda warns him that if he leaves the compound again, she'll contact the oversight committee and end the project.

Sebastian, however, rigs the thermal camera to play the same image of him sleeping over and over again, enabling him to leave the compound. He sneaks over to Linda's apartment, and watches from the fire escape as Matt comes in. Sebastian, pissed that his colleague is screwing his ex-girlfriend, smashes her window and runs off.

Back at the compound, an invisible dog is constantly barking, and Sebastian, getting sick of the barks, reaches in the cage and smashes the dog against the cage wall, killing it. Sarah once again suspects Sebastian, but his camera shows he was there all night. However, they find the circuit board he rigged the camera with. Knowing he's dangerous, Linda and Matt finally tell them the test was unauthorized, and that they are going to see Dr. Kramer to cancel the project and arrest Sebastian. Unfortunatly Sebastian watched the whole conversation.

At Kramer's house Matt and Linda informs him of the test, and Kramer tells them to go clean out their desks. They leave, and Kramer goes to call the other commitee members, but the phone goes dead. Kramer goes outside, but is confronted by Sebastian, who drowns him in his pool.

The next day, Linda is still waiting for Kramers call, but is told that he is dead when she calls. Then the phone goes dead. And the elevators are locked off to everybody but Sebastian. Preparing to go track down Sebastian, the team members grab their dartguns and thermal goggles and go to search for him, except for Janice. She forgets her goggles and gets left behind. As she leaves the room, Sebastian kills her, so of course, Janice, the only black member of the team is killed off first.

The others notice that she's not with them, so they head back to the office room. There they find Janice, strangled to death. Since one else knows about him, they know he is going to kill them to keep the secret. To find him, the others check motion detectors in the hallways and lock off the corridors from the rest of the lab. Matt and Carter are sent into the hallways to track him down, in a scene reminiscent of Alien. By reminiscent I mean ripping it off shamelessly. Matt finds a large heat signature behind some pipes, and shoots it, only to find out it was a steam vent. However Carter is lifted in the air by Sebastian, who was on a set of pipes above. Matt shoots at him, but only hits some pipes behind him. Sebastian drops Carter, smashing his head against a pipe junction box, injuring him severely. Sarah goes to get some blood for Carter, and is confronted by Sebastian in the medical room. Throwing around some blood on the floor, she tries to leave, but is thrown back. Desperate, she throws blood into the air in front of her, and outlines the shape of Sebastian. Sebastian then shoots her with her tranquilizer gun and snaps her neck.

Meanwhile, Frank who was watching over Carter notices he's dead, and leaves with Matt and Linda to find Sarah. They find her dead in the medical room. Frank turns his back to the door, and is impaled by Sebastian with a crowbar, then hits Matt in the stomach with the curved end resulting in a nasty stomach wound. He then locks them in the walk-in refrigerator, turns down the temperature and leaves.

Sebastian, in an attempt to cover up the killings, makes nitro (yeah, right!) and puts it in a centrifuge, and sets the timer for five minutes.

Linda, desperate to get out of the rapidly freezing fridge, constructs an electromagnet with a difliberator and a door handle and opens the door lock. Then Linda, holding a home-made flame-thrower finds Sebastian trying to leave through the elevator, toasts him. She then hits the sprinkler system, and watches for him. He surprises her and slams Linda against the wall. But Matt shows up and smashes the crowbar against his head knocking him out. Linda and Matt walk away, but Sebastian gets up, grabs the crowbar, and tries to hit Matt, only to have Matt duck and the crowbar electrocute Sebastian. The electricity some how makes Sebastian part visible, but they leave him behind. In the lab, they find the centrifuge fifty seconds away from activating, and since they know the stuff is nitro and that Sebastian smashed the controls, they run to the elevator, where they leave through a maintenance hatch.

They get to a considerable distance up the shaft on a ladder before the lab explodes completly, sending the elevator car up the shaft right past them, and then back down, snapping away from its supports. Linda and Matt climb past the stopped car but Linda is pulled onto the top of the car by Sebastian, who somehow survived the explosion.

He goes to kiss Linda one last time, and Linda uses the opportunity to press the cable release button, sending Sebastian to his death in the pit of fire below. Matt and Linda finally reach the top and are rescued by firefighters called to the explosion. And mankind is safe, at least until Hollow Man 2!

"Great acheivements are made not by following protocol, but by seizing the moment."-Sebastian, telling Linda and Matt why he lied to the pentagon. Personally, I thought this was how people ended up in jail.

When Sarah wakes up after Sebastain plays with her, her breast somehow pops back into her shirt, yet she woke up right as he was playing with it.

The human eye works by light reflecting in through the cornea into the retina and the brain processes sight. Yet the invisiblity works by light passing through the person, meaning that Sebastian should have been blind, but a blind invisible man stumbling around wouldn't have been very frightening.

When Linda and Matt find the centrifuge full of nitro and the broken control panel, they just run off, instead of, oh maybe... pulling the power cord out! God, for scientists they sure are stupid.