Darien Fawkes, a thief, is caught while trying to rob an elderly man's safe and, because it's his third strike, is sentenced to life in prison. However, as he waits to be transported to prison, his brother, Kevin, visits him and offers to get him out if he will participate in a secret experiment that the U.S. government is conducting. Darien agrees grudgingly and is transported to a secret military installation in the desert. There, scientists, led by Kevin and a Swiss-French scientist named Arnaud, install a synthetic gland in Darien's brain which causes him to secrete "Quicksilver," a compound that bends light and therefore has the power to make him invisible. But as they fine-tune their experiment, Arnaud deviously tries to steal the necessary files. When Darien catches him and tries to expose him, Arnaud blames it on "Quicksilver Madness," a side-effect of the gland. But, before any investigation can start, a group of Arnaud's mercenaries bust into the compound, stealing the information on the gland and spiriting Arnaud away — and leaving Kevin dead.
Darien is able to escape, but he knows it's only a matter of time before he actually goes insane. He searches out his ex-girlfriend, Casey, a doctor, who is furious at him because he never revealed to her before he was arrested that he was a thief (he claimed he worked for UNICEF). He tries to convince her of the government experiment, the spies, his invisibility, but she doesn't believe anything he says. Before he can press his case, though, he is taken away by men in trenchcoats and brought to a CIA hideout where he meets The Official and Eberts, the top two operatives on the invisibility case. They reveal to him the truth about Arnaud (he's a terrorist financier) and propose to Darien that, if he wants to live, he'll have to go to Mexico, where Arnaud and his people are trying to replicate the Quicksilver gland.
Having no choice, Darien heads down to Mexico, where he meets up with Hobbes, a washed-up but still heavily armed James Bond spy type. After a shoot-out with some French-Canadian undercover spies, Darien "Quicksilvers" and manages to get himself and Hobbes out of there alive. Meanwhile, Arnaud lures Casey down to Mexico on the premise that he works for the CIA and only she can save Darien's life. When she arrives there, Arnaud and his accomplices capture her and use her as bait for Darien and Hobbes.
This, of course, forces the issue in Darien's mind, and he goes directly to Arnaud's compound. Using his invisibility powers, he's able to elude the security system and wreak enough havoc to get to Arnaud and prepare to kill him. But, just before he does, Arnaud reveals that there's a "counteragent" to Quicksilver Madness — and if Darien doesn't get an injection of it frequently, he'll die. Arnaud tries to get Darien to join his side, but Darien refuses, and, before long he and Casey manage to escape, blowing up the compound in the process. But the counteragent and the formula to recreate it have been lost, and Darien knows he'll soon die.
Back at Kevin's gravesite, Darien has a quiet moment, knowing he'll soon join his brother. But he's surprised to see The Official there, who makes Darien an offer: work for the CIA and they'll supply him the counteragent — for which they have captured the formula — when he needs it. It's an offer he can't refuse.
Note: The above synopsis describes the agency that owns the Quicksilver project as being the CIA. This is incorrect. The agency was simply known as The Agency.