Guinevere Pettigrew (FRANCES McDORMAND) is a late 1930s era governess with a reputation for being difficult, which means she's just been fired. With only the clothes on her back, she desperately needs another job from an employment placement service, but she's turned down. Desperate, she spots the name and address of a needy client, Delysia Lafosse (AMY ADAMS), and heads to her place in hopes that she'll find work with her.
It turns out Delysia is a cute and charming but high-strung American socialite who's in London hoping to become a notable starlet. She's certainly had no problem attracting men, a point Delysia quickly learns as the young woman begs for her to help in juggling them. There's Phil (TOM PAYNE), the son of a theater producer who's putting on his own play, and Delysia knows she must battle Charlotte "The Rabbit" Warren (CHRISTINA COLE) for the lead part.
Delysia helps hurry him out of the flat just in time as nightclub owner Nick (MARK STRONG) arrives, desirous of a bedroom liaison with his young lover, but Miss Pettigrew likewise manipulates the scene to prevent that from happening. Then there's Michael (LEE PACE), a poor piano player who performs onstage with Delysia and wants her all to himself.
Delysia is impressed by Miss Pettigrew's ability to improvise and deal with all three men, resulting in her introducing her to her friend Edythe Dubarry (SHIRLEY HENDERSON), the high-society fiancée to clothing designer Joe (CIARAN HINDS). Edythe provides Miss Pettigrew with a complete makeover, but in exchange wants her to smooth things out with Joe over Edythe's scandalous behavior.
As the night wears on, Miss Pettigrew finds herself caught up in a world to which she is unaccustomed, but also one in which she learns she can fit in quite nicely, even if she's bluffing her way through most of it.