For a while in Jamie Meltzer's mesmerizing documentary Informant, I wondered whether subject Brandon Darby, the lefty activist turned FBI informer, was being played by an actor.

But no: It's Darby, and he's a handsome fellow, with haunted eyes blazing out of a bone structure to die for, and with a Montgomery Clift dimple in his chin. Staring straight into the camera, he testifies with the intense calm of a messiah or a madman, which all too often comes to the same thing. Among other things, this powerfully confused man is a study in American extremity.

Like almost everyone else in the crowd of fellow travelers and detractors who come out for and against him — sometimes both — Darby is more unreliable narrator than cynic. Certainly he comes across as a grandiose believer in his own propaganda, whether as the founder of the militant grass-roots aid movement Common Ground, created as a response to support survivors of Hurricane Katrina, or as the disillusioned turncoat and FBI plant who egged on, then turned in a group of young hotheads plotting to disrupt the 2008 Republican Convention in Minneapolis.

How Darby got from A to B is a both a shorter journey than you might imagine and a longer one. Meltzer, a filmmaker with an inquisitive nose for complexity and contradiction, spends just enough time on Darby's Texas childhood and distant-but-domineering dad to conclude that they pretty much programmed him for the role of rescuer — and for the crippling 50-50 ambivalence toward authority that made him first a dissident leader and then a slavish follower of FBI orders.

According to a former associate, it was Darby's savior complex that moved him to rush south to aid a Katrina-imperiled friend, then stay in New Orleans to help out other survivors and found the Common Ground . And perhaps his difficulty with father figures caused him to tell a police officer who asked what he was doing in the area that "I'm trying to foment social change" — then later bond so deeply with an FBI handler that he readily sold out an amateur cell of would-be terrorists trying to rig up Molotov cocktails using tampons.

Perhaps, too, it was Darby's insatiable hunger for attention and recognition that allowed him to participate in Meltzer's reenactment of that very betrayal — an Errol Morris-style dramatize-it gambit that made me squirm a bit, and one which pretty much convicts Darby of entrapment.

In part, his conversion plays out not just as the tale of a tragically vulnerable lost soul, but as black comedy. One former colleague splutters with rage as he describes Darby's efforts to get him to man up by ordering him to stop eating tofu. And as the Minnesota journalist who outed Darby as an FBI stooge gleefully observes, a bunch of incompetents browsing Wal-Mart for bomb parts smacks more of farce than of thriller.

Yet it was precisely that hamfistedness that made that batch of would-be bombers dangerous. And as Darby not unreasonably tells it, he had grown genuinely alarmed by the violent rhetoric of the people he ran with in New Orleans — white racists and former Black Panthers who were arming up amid the post-Katrina chaos.

To his mostly embittered associates on the left, Darby is variously a fallen hero, an attention-seeking egomaniac and a full-on Judas. Absolutist that he is, Darby now excoriates the FBI for insufficient vigor in prosecuting child prostitution. Today he's a Tea Party darling and a passionate columnist for the website Breitbart.com.

Meltzer isn't out to pass any overt judgment; he wants to show Darby as a man of many parts — and, in his way, an all-American. Without a doubt, his subject is given to hurtling between extremes.

Yet the same gifts for paranoia, conspiracy theory and Messianic fervor that launched him on the margins also drew him to the mainstream, where he found allies, enablers and exploiters — among them the New Orleans police chief who fingered him as dangerously anti-government, then brokered his introduction to the FBI.

Where, for a while, he fit right in. (Recommended)

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