Comprenez-Vous Woody Allen's French Version? The metabolism of Julie Delpy’s biting romantic comedy "2 Days in Paris" runs full tilt from the opening scene, of lovers asleep on a fast train from Venice. Once they awaken, the pace accelerates even further, and you strain not to miss a word of their smart, revealing and frequently crazy repartee.

Ms. Delpy stars in the movie, which she wrote, directed and scored, as Marion, a Paris-born photographer who lives most of the year in New York with her boyfriend, Jack (Adam Goldberg), a shaggy, heavily tattooed interior designer. Ms. Delpy intermittently narrates the movie in a voice that is partly hers and partly that of her neurotic, possibly autobiographical character.

After their Parisian jaunt the couple, both in their mid-30s, plan to return to New York. But their tumultuous sojourn in Paris throws into question the future of their volatile two-year relationship.

During their brief visit they stay in Marion’s funky one-bedroom apartment directly upstairs from her parents. Her hyper-emotional mother, Anna, and jolly, gone-to-seed father, Jeannot, are played by Ms. Delpy’s actual parents, Marie Pillet and Albert Delpy. They’re every bit as talkative and excitable as their daughter and her boyfriend, who rarely shut up.

Over his first dinner with his possible future son-in-law, Jeannot tests Jack’s knowledge of French writers by dropping a list of names in which he teasingly inserts that of a painter, Auguste Renoir. Jack is sickened by the rabbit dinner Jeannot prepares.

As the movie pores microscopically over Marion and Jack’s relationship, it reveals more specific information about their habits, tastes, personality traits and emotional and sexual chemistry than any film about a relationship that I can recall. In some ways Marion and Jack resemble a younger Diane Keaton and Woody Allen, if Mr. Allen looked more like Tony Roberts. But even Mr. Allen and Ms. Keaton’s funniest romps came with a protective layer of witty shtick; they never gave you the intimate sense of actually having to live with them night and day.

Jack is a classic neurotic: hypochondriacal with a cleanliness fetish and a susceptibility to migraines. He is as fixated on the possibility of imminent terrorism as Mr. Allen is on the prospect of his own eventual death. Jack, who doesn’t speak French, becomes uneasy whenever Marion lapses into her native language. Because she is a shameless flirt, he imagines all kinds of plots are being hatched behind his back.

Ms. Delpy’s startling resemblance here to the young Ms. Keaton extends from her beautiful, sharp-featured face and frizzy hair and glasses to her piping vocal timbre and nervous body language. Marion is extremely high-strung and sexually willful. Rabidly left-wing, she has no patience for those who don’t share her politics. She physically attacks an ex-boyfriend she meets by chance at a restaurant and loudly accuses him of sleeping with 12-year-old Thai prostitutes when he was in Asia.

A racist cabdriver’s remarks about Romanians and Arabs inflame her, and they get into a shouting match that threatens to turn violent. She uses her political passions to bludgeon Jack during discussions of their sex life, loftily reminding him that their erotic peccadilloes are trivial in light of the world’s ills.

Gradually, stealthily, “2 Days in Paris” turns its gaze from Jack (as seen through Marion’s critical eyes) to Marion (as seen through his increasingly paranoid ones). In his whiny self-absorption Jack initially appears almost unbearable, redeemed only by his sense of humor. But as his confidence falters, you begin to feel for him.

Over two days Marion encounters three ex-lovers and laps up the ogling gazes and come-ons of various handsome strangers. When Jack questions her about who did what to whom and when, Marion airily dismisses her former relationships as no big deal. Jack, who hears conflicting stories about her past, is not so sure. His suspicions coalesce into a despairing paranoid fantasy of Marion as a selfish, lying slut. The little white lies she tells to protect his feelings loom to him as major betrayals.

“2 Days in Paris” is an inside-out version of the much-admired Richard Linklater films “Before Sunrise” and “Before Sunset,” in which Ms. Delpy and Ethan Hawke portray a French-American pair who meet, part and reunite years later. Where Mr. Linklater’s movies were weepies for the kind of educated, upscale young cosmopolites who have a soft spot for romances like “Casablanca,” Ms. Delpy’s examination of modern love among the almost young and still restless is bracingly hard-headed.

Ms. Delpy’s and Mr. Goldberg’s performances are so assured and spontaneous that they don’t even seem to be acting; they’re living their roles in front of our eyes. As they tear each other to shreds, “2 Days in Paris” becomes a meditation on the unknowable essence of another person, even an intimate partner. Audacious as it is, the movie is also a little scary. The chances that either Marion or Jack will one day turn to the other with misty eyes and murmur, “We’ll always have Paris” seem slim.