As the film opens, Miss Pettigrew (Frances McDormand) has been let go by her latest employer. Her specialty is childcare; however, her strong-willed nature does not interact well with the highbrow society within which she works. Due to this conflict the service that employs her tells her that there are no current assignments for her, nor will there be any in the future.

Lingering a little, she witnesses a business card belonging to a potential employer placed, unguarded, on the desk before her. Going against her own strong moral code, she takes the card and leaves. Before taking action on the stolen card, she must take care of herself. She has no money, no place to sleep, and the only prospect is the card in her pocket. So, after a lonely night on the streets the screwball comedy picks up and she finds herself in the foyer of the flat containing a manic Delyssia Lafosse (Enchanted's Amy Adams).

Their first meeting is a mixture of mistaken identity, double entendre, and quick, perfectly timed lines that build to a fever pitch. Miss Pettigrew has arrived at a crucial moment in Delyssia's day. You see, she is something of a golddigger, juggling three men for three different goals, and currently she has to get one out of the house before a second arrives. She enlists Miss Pettigrew as an accomplice in this malfeasance.

As it turns out, Miss Pettigrew could not have arrived at a better time, not for Delyssia, nor for herself. Both women are reaching crucial moments in their respective lives, a turning point where they need to move onto the next chapter of their lives. Each of them is fast approaching the point of no return, although neither of them ar quite prepared to deal with it.

Following their initial encounter, Delyssia hires her on to be her social secretary. Neither is quite sure of what the position entails, but the two have bonded so quickly that they go along with the flow. The further into the day they go, the closer to a life-changing moment they get.

Now, this all sounds awfully serious, but it is anything but, even with shadow of the impending second World War. These women find themselves entangled in love triangles, each with their own comical complications.  Delyssia is juggling Nick (Mark Strong), the night club owner whose riches attract her and who provides her with employment and an elegant lifestyle; Phil (Tom Payne), the young stage producer set to cast her as the lead in his latest production; and Michael (Lee Pace), a poor piano player. On Miss Pettigrew's side there is the cold, manipulative Edith (Shirley Henderson), and her on again/off again fiance Joe (Ciarin Hinds), a lingerie designer who is quite taken with the rather frumpy Miss Pettigrew. Together, Delyssia and Miss Pettigrew must find themselves and help each other to that end.