(FORTUNE Small Business) – Okay, so he refuses to file a Form 1040. But Joe Banister, 48, is not an obvious kook. Nor is he a "miscreant, knuckle-dragging mountain dweller" (his words--he knows what people must think). He insists, furthermore, that he is not a Libertarian, or even a libertarian. How could he not believe in government? His late father worked as a building inspector for 30 years for the city of San Jose; his brothers are two cops and a firefighter. What's more, after a stint at Peat Marwick, Banister became a special agent in the criminal investigation division of the IRS, his dream job. He packed a SIG-Sauer 9-mm, which he used while chasing drug smugglers and money launderers. "I thought it was neat to be able to use financial skills to investigate criminal activity," he says. "I thought it was the last thing that a crook would expect."

No. The last thing a crook would expect was that Banister would cross over to the dark side. Banister resigned from the IRS in 1999, shortly after presenting his superiors with a meticulously researched, 92-page report attacking the legality of the federal tax code and the means by which it is enforced. The catalyst, he says, was an interview he heard on the radio one day with Devvy Kidd, whose Project on Winning Economic Reform, based in Sacramento, aims "to educate and motivate Americans about the mechanisms bringing America to ruin." Banister later read Kidd's pamphlets. One line jumped out at him: "IRS agents are insults to the word slime." "I thought to myself, I'm not slime. Why is she saying I'm slime? It really stuck in my craw." So began an obsessive two-year investigation into Kidd's claims, at the end of which, sad to say, Banister decided that Kidd was right.

The questions Banister raises are not new: Is the income tax unconstitutional because the 16th Amendment was never properly ratified by the states? Does the IRS violate the Fifth Amendment when it requires us to give potentially incriminating information on our tax forms? Do IRS agents routinely abuse their authority? The same questions have been raised repeatedly by kooks, miscreants, and libertarians ever since the ratification of the 16th Amendment ("Congress shall have power to lay and collect taxes on incomes") in 1913, and dismissed repeatedly by the courts. Which is probably why, initially, Banister was ignored.

But Banister pressed his case, insistently and authoritatively, to anyone who would listen. He spoke at seminars for small-business owners, complained to lawmakers, and logged more than 150 radio interviews. Ultimately, the government alleges, he began advising clients (one of whom, Walter Thompson, was convicted on 13 criminal tax-evasion charges in January) on the hows and whys of organized tax resistance, at which point the IRS decided enough was enough. On Nov. 18, Banister was arrested at his home in San Jose and charged with conspiracy to defraud the U.S. He faces 18 years in prison and a fine of as much as $1 million. A trial is scheduled for June.

A product of 12 years of Catholic schooling (the last four at an elite prep school run by Jesuits), Banister is nothing if not pious. "I took an oath to support and defend the Constitution," he reminds me over a lunch of braised chicken with mushrooms at the Coquette Café in Milwaukee, near his Wisconsin lawyer's office. "I take my oaths seriously." Twice during our meal he falls silent and, gulping, fights back tears. "If I were to stay silent about this issue," he says, "I would be one of the biggest hypocrites that ever walked the face of the earth."

No, Joe, I'm thinking on the plane ride home, the hypocrite would be the priest you mentioned as I was leaving--the one who tried to molest you when you were a kid. If you change paths now, you won't be a hypocrite and you won't be betraying anybody's trust. Just the opposite: You'll be an honest Joe. Somebody who pays his taxes and plays by the rules, even when those rules don't seem fair.