Ironically, it’s the first half of “Talk to Me” that cripples its final act, building up expectations with its energized and electric performances (not to mention its scathing social criticism) before proving unable to live up to them. Watching it sputter and careen into the familiar form of your standard biopic, one immediately sees the flaws of this whole genre; if only “Talk to Me” had ended a bit sooner, on a ragged note, without tying up all the loose ends, it could have avoided its demise. If it’s a true story, why must it feel so Hollywood?

The energy jumps off the screen from the very beginning of the opening credits as Ralph “Petey” Greene (Don Cheadle) sarcastically preaches and cracks wise on the prison airwaves to his fellow convicts. Locked up for five-to-10, Petey has embraced the opportunity to commandeer the prison radio microphone, seeing it both as an opportunity to earn early release and a way to pass the time. When Dewey Hughes (Chiwetel Ejiofor), a Washington, D.C. radio manager, visits the jail one afternoon, his brother tells him to watch out for Petey — that he’s the real deal — and Dewey tells the prison DJ to look him up when he’s back on the outside.

Thus begins a most bizarre friendship, a love-hate relationship that at first pits the vulgar ex-con against the convertible-driving professional. Soon, their back-and-forth hints at the larger conflict at the very heart of “Talk to Me,” a conflict within the black community that we have seen erupt anew with the presidential campaign of Barack Obama: How black is black enough?

The more Dewey scoffs at the notion of giving Petey a job—maybe the job of a janitor, Dewey says to the ex-convict, or a window washer — the more Petey scoffs at this business executive, mocking his “white sounding” voice, calling him Uncle Tom, and comparing him to Sidney Poitier.

It’s a polarizing issue in the mid-1960s when this story begins, against the backdrop of Dr. Martin Luther King, Jr. and the Civil Rights movement. Dewey, from the outset, dubs Petey a “thug,” a “waste,” and a “miscreant”; says that he’s “just another convict,” and that he always will be. Petey retorts that Dewey has turned his back on his people by refusing to give a brother a break after the horrors of prison, and that everything about his existence has been tailored to mimic the white man.

It’s precisely for this reason that Dewey gives Petey the opportunity one day to get on the air and prove his mettle. Not that he has much of a choice. As the station’s ratings plummet and WOL, “the people’s station,” watches its demographic fade, Dewey recognizes in Petey a connection to a world he knows nothing about. Putting Petey on the air was a risk, but when he shows his bosses the flooded phone lines of people calling in, responding to this new raw voice of Washington, it becomes clear that Petey’s controversial morning banter — and Dewey’s business sense — have become the hit of the city.

Day in and day out, backed by an electrifying soundtrack, Petey does more than run through a list of obscenities; he confronts the issues of the local black community, expressing outrage over the murder of a young black man by D.C. police and mocking the producers of Motown music, dubbing them a special breed of pimp.

But “Talk to Me” wouldn’t be a worthwhile story if Petey’s story of triumph were as simple as all that. Having found success on the airwaves, he has also found a celebrity that threatens to alienate him from the very people he came to represent. How can he remain a man of the streets when he starts doing stand-up routines to sold-out white comedy clubs, or appearing on “The Tonight Show Starring Johnny Carson” — the apex of “mainstream” celebrity.

This is what ultimately wedges Dewey — the man of the establishment—and Petey—the man of the people — apart, and what precipitates the movie’s clunky and uninspired conclusion, a generic denouement that seems to lose momentum by the minute. It’s here that Mr. Cheadle, who delivers an infectious and exuberant Oscar-worthy performance, and Mr. Ejiofor, who elevates “Petey’s buddy” into his own fully realized and conflicted character, find themselves stranded by a script that loses its edge and nerve, preferring instead to go down the road of caustic arguments and contrived reconciliations.

It’s these two brilliant actors who make “Talk to Me” something greater than a simple buddy story. As Dewey recognizes the shifting American tides early on, and watches as they unite at the right time for the right cause, we are privileged to watch with him. That the film ends so predictably is unfortunate. Indeed, before “Talk to Me” starts talking at us, it bristles with the kinetic spark of that rare story about a remarkable person achieving something great against historic odds.