Exactly the sort of period comedy piece that people like to call “effervescent,” “Miss Pettigrew Lives for a Day” is long, long, long on the dual charms of its leads, Amy Adams and Frances McDormand, whose yin-and-yang chemistry gives this breathless contrivance its bubbly pep.

Once again, Ms. McDormand, who can radiate a dangerous sexuality (see “Laurel Canyon”), is called upon to dowdy up as the titular miss, a stiff governess named Guinevere, whose Arthurian mojo has long gone missing amid the menopausal fog. As the movie opens, she’s getting the heave-ho from her latest disastrous job assignment, caring for some upper-class twit’s spoiled brats. Penniless and starving, Miss Pettigrew can’t even juggle a bowl of soup-kitchen gruel without a bum knocking into her and spilling it. As the harsh wind whistles through the dank, cobbled alleys of 1939 London, Guinevere clutches her threadbare sweater and wobbles toward the train station for shelter, a feminine evocation of Chaplin’s eternal Little Tramp.

But she still has some wit about her. A seemingly ill-timed visit to her employment agency wins her the unceremonious boot, but also allows her to snatch a name and address off a tabletop, thus intercepting an opening as a “social secretary” for a young American actress, Ms. Adams’s Delysia Lafosse.

Much screwball hither and dither ensues, during which Guinevere sorts out the promiscuous Delysia’s complex dating schedule — seems there’s a young dandy naked in the master suite, and the starlet’s playboy patron has just arrived downstairs — and the two women discover their symbiotic need for each other. Despite appearances, they’re both dependent on the kindness of strangers. And as England prepares for World War II, the heedless frivolity of the day is about to come to a jarring halt. But not just yet.

Working from an adaptation of a 1938 novel by the proto-feminist writer Winifred Watson, director Bharat Nalluri (“Tsunami: The Aftermath”) revels in time-capsule chic. He maximizes the film’s central location, a post-Art Deco townhouse, as one would a Broadway stage, punctuating the actors’ busily choreographed movements with the swoosh of swinging doors and the camera’s agile appreciation of extravagant costume and set design.

Ms. Adams, who is meant to emulate a kind of nostalgic floozy, imbues her character with a sense of desperation and dignity that connects her to Ms. McDormand’s diamond in the rough. Ms. Adams’s parents apparently saw to it that she had excellent dental hygiene as a child, as her big, bright smile becomes a most useful tool of serial deception. Ever in hyperdrive, the actress may be trying too hard to play a character who tries too hard, which is understandable since the unsinkable Delysia (an assumed name, it turns out, for a simple girl from the Midwest) is also juggling a third swain (a pianist played by Lee Pace, of “Pushing Daisies”). All three lovers collide not once, but twice, in a pair of successive set pieces shot in crowded social gatherings — a party, a nightclub — where the long-suffering Miss Pettigrew discovers that she, too, may yet find love.

The film’s pleasures will chiefly appeal to fans of the BBC, for whom such throwback reverie is the bee’s knees. As the respective ditz and frump, both Ms. Adams and Ms. McDormand lend their peculiar grace to the theatrical goings-on — a minor respite between assignments that, with any luck, won’t insist they stick so closely to established type.