As F. Scott Fitzgerald famously wrote, there are no second acts in American lives. Somebody forgot to tell Clint Eastwood, who, at 74 and well into his third act, is doing the best, most assured work of his career. And he's doing it in his usual quick, thrifty, no-fuss manner. A year after the dark, epic "Mystic River," he weighs in with "Million Dollar Baby," set in the nether regions of the boxing world. The focus is tighter, more intimate: Eastwood plays a guarded, solitary gym owner and fight manager in Los Angeles who reluctantly agrees to train 31-year-old Maggie Fitzgerald (Hilary Swank), an Arkansas redneck who has nothing in her life but her fierce determination to fight.

This is a setting, and a setup, that comes booby-trapped with fight-movie cliches stretching from "Girlfight" and "Rocky" back to the '30s. This is not, however, Eastwood taking one of his genre-movie breathers; it may be a smaller movie than "Mystic River," but it's every bit as uncompromising, and even more powerful. Think you know where this story is headed? You don't, and I'll do you the favor of not telling. Let's just say that just when you think you've got its number, "Million Dollar Baby" blindsides you with a devastating hook.

As an actor, Eastwood has rarely taken on a character as complex as Frankie Dunn, a gruff, guilt-ridden Roman Catholic who attends mass every day, driving his priest crazy with his incessant, irreverent questions. Frankie broods over the estranged daughter he drove away years earlier. His only confidant is Scrap (Morgan Freeman), a battle-scarred former fighter who looks after the Hit Pit, Frankie's old-style gym. Frankie studies Gaelic, reads Yeats, and doesn't like to take chances. He'd rather hold his fighters back from taking a title shot if there's a risk they might get hurt, and he pays for it when his charges inevitably abandon him for greener pastures. No one has lured him out of his shell until Maggie comes along, and it's obvious the love that grows between them is the love he couldn't give his daughter. In front of the camera, Eastwood's often coasted on his iconic persona; here he inhabits his character completely. Playing a prickly, haunted man who always keeps his guard up, Eastwood gives his most daring, emotional, unguarded performance.

Swank, who was extraordinary in "Boys Don't Cry," hasn't fared so well in conventional leading-lady roles where she tends to disappear. Extremity becomes her. As Maggie, she pops off the screen, funny, touching and ferociously physical. Swank makes us feel the exhilaration that courses through Maggie's lithe body once she learns proper boxing technique and unleashes the knockout blows that send her opponents crashing to the canvas. Her body language shouts.

The well-observed details of the fight world come courtesy of F. X. Toole, whose story in "Rope Burns" is the basis of Paul Haggis's screenplay. Haggis has done a terrific job of remaining faithful to the text while fleshing out the characters. Freeman's Scrap, who narrates the story (OK, maybe there's a little more voice-over than is necessary), doesn't exist in Toole's short story. He's a savvy addition, giving Frankie a sounding board--and Freeman a wonderful role. Margo Martindale is brilliantly loathsome as Maggie's white-trash mom, a woman of chilling selfishness.

"Million Dollar Baby" could have been sentimental or quaint, but Haggis and Eastwood are made of sterner stuff. In his clean, unhurried, unblinking fashion, Eastwood takes the audience to raw, profoundly moving places. If you fear strong emotions, this is not for you. But if you want to see Hollywood filmmaking at its most potent, Eastwood has delivered the real deal.