The operative word in the phrase "based on a true story" is usually the first one. Real lives are always souped up for cinematic consumption, often to a astonishing degree, like the way former Long Beach, New York, cop Vincent LaMarca's has been for the film "City By the Sea."

LaMarca's true story is that his father was executed for the kidnapping, ransom and murder of a baby in the 1950s, yet he grew up to join the police force under the wing of one of his father's arresting officers. Then after he retired to Florida, his own estranged son was arrested and convicted of a ruthless murder.

But in this movie -- inspired by an article about the LaMarcas in Esquire magazine -- Vincent (played by Robert De Niro) is a Manhattan homicide detective whose most recent investigation leads him to his own drug-addled son Joey (James Franco), who accidentally killed a drug dealer in a brawl. A girlfriend (Frances McDormand) and a grandchild have been added to beef up the plot, and so has the murder of another cop by the drug dealer's boss, out to avenge himself on Joey.

Usually such Hollywood embellishments are transparent and trite. But writer Ken Hixon ("Inventing the Abbotts") and collaborator/director Michael Canton-Jones ("Rob Roy," "This Boy's Life") make their fictionalized version of LaMarca's story so compelling and somberly, realistically gritty that it's surprisingly easy to forgive the film's fabrications.

Although Vincent could be seen as a variation on some of De Niro's past roles, the actor gives a more nuanced than usual performance of heavily guarded fervency. Watching him, you can feel how much he loves being a cop, how much he regrets not being a part of his son's life, and how much it hurts him to say (unequivocally despite the circumstances), "If he's a murderer, he's a murderer. It's his choice. He's got no one to blame but himself."

The film is in part about Vincent -- suspended from duty because he's suddenly a public relations nightmare -- trying to bring his son in before the police find him. They think Joey killed the other cop, and Vincent knows that could mean his son is in more danger from the law than anyone else. But it's also about the way Vincent and Joey's hearts have both decayed like the formerly bustling Long Beach boardwalk where the younger LaMarca lives, holed up in a dilapidated casino. And at its heart it's about Vincent trying reconnect with locked-away feelings and reclaim his past.

McDormand ("Fargo," "Almost Famous") is, as always, subtly, fantastically three-dimensional in a supporting role as the woman in Vincent's life, who gives him small shoves in the direction he's afraid to go emotionally, even while coping with a whole lot of new information he's withheld. In one conversation he reveals he has a son, an ex-wife who accused him of beating her and poisoned Joey's mind against him, and a history of unfulfilling supervised visits when Joey was a boy. "Sorry I didn't tell you," Vincent says matter-of-factly. "And I think he might have killed a guy the other night."

Handsome young method actor James Franco (TNT's "James Dean") -- whom I once dismissed as an "interchangeable teenage looker" (I was wrong) -- adds another layer of intensity to the film with his complete immersion in the strung-out, screwed-up life of Joey LaMarca. His honest desire to claw his way out of the hole his life has become shows through profoundly, even as he skulks along the trash-strewn boardwalk, trying to sell his guitar so he can freebase his miseries away.

Franco may be a little pouty and seems to take himself a little too seriously, but whenever he's on screen in "City By the Sea," the emotional ante goes way up -- especially when he and De Niro finally come face-to-face.

Director Canton-Jones never lets go of the raw nerve the film exposes, making the audience understand viscerally each of the characters' sense of being overwhelmed by their circumstances, and it's a powerful experience to see De Niro break down his stony facade in Vincent's eventual emotional watershed moment.

Because "City By the Sea" paints such an authentic, vivid picture of the fictional Vincent's life, most of the film doesn't feel conspicuously scripted. The ending, however, just screams "Hollywood" as it wraps everything up in a too-neat little package that conveniently allows for redemption, reconciliation, heroism and accountability all in one fell swoop. But this last-minute contrivance is not enough to tarnish the solidly substantial and intelligently dramatic -- if almost entirely imaginary -- experience that has come before it.