Miss Pettigrew Lives for a day (PG)

It's lucky that Amy Adams and Frances McDormand lend their best charms to Miss Pettigrew Lives For A Day, otherwise this featherweight Art Deco fancy might drift away like a soap bubble, and go 'pop!'.

It's set in London, 1939, just as the stormclouds of war are gathering – the sort of heritage piece that celebrates the British spirit of soldiering on and making do. And, wouldn't you know, it requires two Americans to parachute in and save us.

Another small rescue story lies behind its production. The film is based on a novel of the same name, by Winifred Watson, first published in 1938 and quickly sold to America and France. Germany also wanted a translation, but war intervened. Persephone Press brought it back into print eight years ago.

So Miss Pettigrew lives again, and one can see why film-makers would be attracted to her story. For one thing, it's an adult fairy tale whose action is confined within 24 hours. (Its first chapter is subtitled "9.15am – 11.11am".) For another, it centres upon an ill-starred but virtuous woman who is granted a second chance.

That would be Guinevere Pettigrew (McDormand), a middle-aged governess who, fired from her latest job, has been reduced to seeking her daily bread at a soup kitchen. Her brown macintosh is as dowdy as her frizzed hair, and the set of her jaw bespeaks a lifetime of quiet disappointment. By an untypical sleight-of-hand she finagles an opening as "social secretary", and turns up at the address with no idea as to what the job might entail. Fortunately, her prospective employer has no idea either. That would be Delysia Lafosse (Adams), a scatty, free-spirited American actress who instantly and gratefully drags Miss Pettigrew into her chaotic private life.

The two women could scarcely make more of a contrast: the one a meek English spinster and daughter of a curate, the other a ditzy, cocktail-swilling, gal whose eagerness to please hides a fierce ambition. Occupying the latter's bed is Phil (Tom Payne), toff son of an impresario from whom Delysia is angling for the lead role in a new West End musical.

Miss Pettigrew's first duty is to turf Phil out before another suitor of Delysia's, nightclub owner Nick (Mark Strong), turns up at the palatial rooms he has been renting for her. Surviving this bedroom farce clinches the women's friendship and, as a reward, Delysia takes her bedraggled social secretary ("you look like Oliver Twist's mom") for a girlie makeover, beadily supervised by couture stylist Edythe (Shirley Henderson), who thinks she sees through Miss Pettigrew's disguise.

This brittle comedy of manners rolls merrily, if rather stagily, along, kept buoyant by the watchful nerviness of McDormand and the to-hell-with-it breathiness of Adams, who seems to be channelling a mixture of one part Gloria Grahame and two parts Marilyn Monroe in Gentlemen Prefer Blondes.

The actress on the make feels a wearyingly familiar type, yet Adams brings something quite unusual to the part. The overbright innocence of her young mum in Junebug is married now to a feverish calculation, and she holds these competing character traits in marvellous equilibrium; a slave to instinct and caprice, she seems barely to have a clue about what to do next. You can pretty much guarantee that whatever it is will be accompanied by an impossibly wide smile. But Delysia has entered so completely into a life of deception that she can't see the lies she's telling herself: it's not Phil or Nick she wants but the soulful pianist Michael (Lee Pace), even if he is a penniless jailbird.

The frothiness of the showbiz intrigues is counterpointed by the implicit sadness of Miss Pettigrew's existence. What ghosts beneath the surface glitter are the realisation of a life not fully lived, of jobs precariously held, and the chasm of poverty menacing a certain Englishwoman of the time. Her story spills forth: a sheltered upbringing, a fiancé who went off to fight in France and never returned, then 20 years of dismal to-ing and fro-ing between positions in "service". The problem facing the writers, David Magee and Simon (Full Monty) Beaufoy is that these revelations should emerge by degrees, over a period of weeks rather than a single day.

The appearance of Ciaran Hinds as a suave designer of ladies' underwear tilts the mood nicely towards romance, but his emergence from an unsatisfactory relationship – he is engaged, then disengaged within hours – doesn't have the comic momentum required to pass as screwball.

You may feel inclined to back it, all the same. There is something about its innocent spirit of gaiety that holds a strong appeal, and director Bharat Nalluri works his socks off trying to smooth it along. The creaks in its staging can be forgiven so long as you concentrate on the fabulous production design (by Sarah Greenwood), the costumes (by Michael O'Connor) and the exuberantly brassy score (by Paul Englishby). The rest is down to the playing. In a stronger movie Adams and McDormand might between them pick up an award or two.