When Ahn "Joseph" Cao won election to the House in 2008, he ticked off a couple of firsts. He was the first Vietnamese-American to serve in Congress. He was also the first Republican in more than 100 years to win election in Louisiana's 2nd Congressional District, which covers nearly all of New Orleans. With zero experience in politics (he was formerly in training to become a priest), the diminutive candidate won election in the heavily Democratic district only because his opponent, William Jefferson, was mired in scandal. When he arrived in Washington, he was known as the Accidental Congressman.

Two years later, Cao was out of a job. His unlikely rise and precipitous fall is the subject of Mr. Cao Goes to Washington, a new documentary from PBS that premiered on Thursday night and will be aired across the country by various affiliates in the coming week. (Watch a trailer below.) The film, directed by Leo S. Chiang, is the story of what happens to a well-intentioned rookie who finds himself thrust into the partisan miasma of Congress.

Cao was always something of an anomaly in the country's halls of power. He told Deborah Solomon at The New York Times that he had joined the Republican Party for its "strong pro-life stance. That alone." His positions on other issues were decidedly liberal, though they were inflected with a religious bent. "I always adhere to what I call 'the politics of the Gospel,'" he told Solomon. "You have to take care of the poor, take care of the widows, visit the sick, help those who cannot help themselves." He crossed GOP orthodoxy multiple times (irritated conservatives kept track of his infractions), and, most egregiously, was the only Republican who voted to pass President Obama's health care bill. The distinction earned Cao the president's affection.

Or so he thought. The health care vote upended Cao's life. Republicans accused him not only of betrayal, but of pandering to his mostly black constituents. Cao then switched his vote to "nay" for the bill's final draft, claiming that the Senate had introduced language that could allow federal money to be used for abortions. His about-face earned him the disdain of Democrats, who claimed that Cao had let Republicans bully him into line. Cao claimed that all his votes were reflections of his conscience and beliefs; his opponents on both sides of the aisle said he reeked of political opportunism. And, when the air is so foggy with cynicism, who knows which side is telling the truth?

Cao was especially disappointed when he discovered that Obama had cut an ad in support of his 2010 opponent, Democrat Cedric Richmond. "From the tone of his campaign I expected something different than the usual political stance," he told Carlo Rotella of The Boston Globe. "I had hoped he was a person who would do the right thing, not the expedient thing." As Rotella notes, "Can he possibly be naive enough to mean this?"

Naive, earnest, hopeful: That's how Cao started off his great congressional adventure. What he became is the subject of Mr. Cao Goes to Washington.