IN 1930s BRITAIN, Julia Lambert (Annette Bening) is the reigning queen of the West End. But the stage actress is feeling tired and just a little bit old. When she asks her producer-husband, Michael (Jeremy Irons), to stop her current play, he doesn't pull the plug. Julia can be flighty. She's feeling insecure. She may yet reverse her decision.

Is this the beginning of Julia's next act, when she'll segue from leading lady to supporting, older player? Michael and her dresser Evie (Juliet Stevenson) know to keep a wary eye on her mercurial ways.

In "Being Julia," Julia's life takes a sudden turn when Michael introduces her to a young, bright-eyed American fan named Tom (Shaun Evans). At first she's dismissive of this twenty-something bookkeeper and his almost frantic attention. He blushes in her presence, buys her roses, wants to spend as much time as he can with her.

"It's not the play" that he loved so much, he tells her, as they face each other in an elevator. "It's you."

Julia soon surrenders. His fervor is too strong to resist. He's young and fetching. And her physical relationship with Michael is practically nonexistent. Before long, Tom's persuaded her to come to his modest flat. They're kissing. And she's back, her desire to act renewed. And she's sleeping with someone as young as her son Roger (Tom Sturridge).

Julia is acutely aware, and yet only semiconscious, of the roles she plays. She's a highly theatrical actress for the audiences, with great big gestures. She's a wife to Michael, although there's a gulf between their spirited friendliness and real passion. She's a mother. And now she is, in a sense, playing a young lover with Tom.

Her only frames of reference are her son, also in his twenties, who accuses her of not even existing, and a fantasy figure, the ghost of her former acting coach (Michael Gambon), who comments on the credibility of her various performances, in life and on the stage.

Reality is Julia's most formidable hurdle. She learns a number of disconcerting things: about Tom's real agenda, about the ambitions of a young ingenue (Lucy Punch) trying to get into Michael's company, even about her husband. There are other revelations, too, from her friend Lord Charles (Bruce Greenwood). And her son. Ultimately, Julia takes decisive action in the only way she knows how: onstage, with everyone watching. Will she find her true self? Or is she entirely lost in her processes?

"Being Julia," based on a W. Somerset Maugham novella titled "Theatre," is based on the overfamiliar premise of a self-indulgent actress facing the inevitability of age. But director Istvan Szabo (who made "Mephisto," another film about role-playing, and the less successful "Sunshine'') and screenwriter Ronald ("The Pianist") Harwood have worked a potential sow's ear into a rather attractive purse.

They put a lot of amusing backstage banter into the story, too. When Julia laments that she's "a bitch, awful through and through,'' her husband, witheringly and with love, replies: "Nevertheless . . ."

Bening makes the movie into something finer still. She digs into a pagoda-size heap of roles and roles-within-roles and pulls them all out, one by one, deftly. You feel the fun of the thing, as well as the appropriate heartache. She also has the smarts not to Paltrow around with an English accent but simply speak in a neutral New England/mid-Atlantic voice. You stop paying attention to that and concentrate on the performance instead. This Julia's worth that investment. Instead of being a tiresome diva, she's surprisingly affecting and fragile. And when it does come to vamping it up, her final act is a treat worth waiting for.

BEING JULIA (R, 104 minutes) -- Contains nudity, sexual situations and some obscenity. Area theaters.

Annette Bening brings vulnerability and wit to the role of an aging theater actress grappling with a midlife crisis in "Being Julia."