"Julie & Julia" is one-half of a great biopic. In portraying Julia Child — the effervescent author and chef whose four decades of TV cooking shows made her an intimate presence in millions of American homes — Meryl Streep gives a performance that's virtually an act of reincarnation. One of the film's most striking moments is a scene in which Julia and her husband Paul (Stanley Tucci) are having dinner in a Paris restaurant, and Paul asks his wife what it is she most likes to do. Happily chewing a forkful of food, Julia tosses her head and blurts out, "Eat!" That's all. But Streep's careful blending of gestures and expressions — breezy, ebullient, half-dotty — is so precisely Julian that a smile of wonderment is impossible to suppress.

Streep's accomplishment is remarkable because Julia Child — one of the great American originals — seems so far beyond the reach of non-satirical depiction. She was a towering Californian (6'2" tall) who joined the OSS (precursor to the CIA) during World War II, working for the agency in both Washington, D.C., and China (as an intelligence researcher, not a spy, in her account). At age 34 (still a virgin, she blithely noted), she married Paul Child, a U.S. Foreign Service officer. In 1949, he was assigned to Paris, and it was there that Julia had her first French meal — a simple filet of sole with butter and lemon that changed her life.

Suddenly impassioned, she started learning French and enrolled in Le Cordon Bleu, the celebrated (and, she said, overrated) Paris cooking school. Soon she connected with two Frenchwomen, Simone Beck and Louisette Bertholle, who were attempting to write a book about French cooking for Americans. Julia joined them, and over the next eight years, as she moved with Paul to Marseilles, Bonn, Oslo and finally back across the Atlantic to settle in Cambridge, Massachusetts, she and her partners continued working on the book by mail.

Their final manuscript was a collection of more than 500 recipes — each meticulously researched, translated and repeatedly tested by Julia — that would eventually run to 720 printed pages. More than one publisher shied away from a specialized book of this size, especially since it was commonly assumed at the time that American cooks — meaning American women, for the most part — were quite happy with canned soups and packaged mixes, and had no interest in classical cooking techniques. Julia Child's culture-shifting perception was how wrong this assumption was. The book she created with her partners, published in 1961 as Mastering the Art of French Cooking, was an instant hit, and is now in its 49th printing.

Two years later, a Boston PBS station offered Julia her own cooking show. "The French Chef" started out as a low-budget black-and-white production that showcased not only the star's culinary expertise, but also her cheery knack for on-camera improvisation. In one famous incident, while frying a large potato pancake, she lifted the pan to flip the thing and — whoops — slopped a good part of it onto the stovetop. "That didn't go very well," she muttered, scooping the clumps back into the pan. "But you can always pick it up. And if you're alone in the kitchen, who is going to see?" (A persistent urban legend has Julia dropping a whole roasted chicken or lamb haunch or something on the floor in one show, and lifting it back onto the serving plate. According to Snopes.com, the killjoy Web site, there's no taped evidence that this ever happened.)

The potato-pancake incident makes it into "Julie & Julia," along with much else from Child's memoir, "My Life in France," which was assembled from letters and memories by an admiring grand-nephew, Alex Prud'homme, and published in 2006, two years after Child's death. As in the book, a prominent theme in the movie is Paul and Julia's extraordinary mutual devotion. Paul, a paragon among husbands, it seems, offered Julia unflagging encouragement and endless assistance (in the early days of the TV show, he washed the pots). Streep and Tucci, last paired in "The Devil Wears Prada," have an effortless chemistry that brings this grand relationship — a rare perfect match — to full, glowing life.

Unfortunately, "Julie & Julia" isn't entirely about Julia Child. It was writer-director Nora Ephron's cute idea to pair the story told in "My Life in France" with another one adapted from a 2005 book called "Julie & Julia: My Year of Cooking Dangerously," by Julie Powell. In 2002, Powell, a 30-ish New Yorker trapped in a dead-end office-drone job, started a one-year Salon blog project in which she undertook to cook every recipe in "Mastering the Art of French Cooking." The blog became popular, and the book deal followed. In the film, Powell is fetchingly portrayed by Amy Adams, and there are some amusing moments as she slogs her way through Child's imposing tome. (Tapping at her laptop, Julie tells her online audience, "We are, I am sorry to say, entering aspics.")

But Powell's story is an emaciated thing compared to Julia's. She, too, has a husband (played by Chris Messina), but his support only extends so far, and at one point he leaves her. (In a second book called "Cleaving," originally scheduled for publication this month, but now pushed back to December, Powell recounts a wild extramarital affair.) More important, though, the contrast between Julia Child's accomplishment and Julie Powell's — which could pass for an accomplishment only in the blog world — deflates the movie like a failed soufflé. Child created a book (several in fact) and a public persona that were unique in their time; she removed the intimidation from French cuisine and made it a possibility for home cooks. (She also inspired many a future celebrity chef.) Powell simply slipstreamed in Julia's aura. And her story, a modest online entertainment, is a wobbly element in a movie that might have been better had it concentrated fully on its most fascinating subject.

Check out everything we've got on "Julie & Julia."

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Don't miss Kurt Loder's reviews of "Cold Souls" and "Paper Heart," also new in theaters this week.