The Brown children -- Simon (THOMAS SANGSTER), Tora (ELIZA BENNETT), Eric (RAPHAEL COLEMAN), Lily (JENNIFER RAE DAYKIN), Christianna (HOLLY GIBBS), Sebastian (SAMUEL HONYWOOD) and Baby Agatha (HEBE & ZINNIA BARNES) -- are legendary in their Victorian era London village. Actually, notorious is more like it as they've just proceeded to drive away their 17th nanny. Evangeline (KELLY MACDONALD), the scullery maid, puts up with but can't control them, which also holds true for family cook Mrs. Blatherwick (IMELDA STAUNTON).
Not surprisingly, when the children's beleaguered and recently widowed father Cedric Brown (COLIN FIRTH) goes looking for another nanny, he can't find one. And with most of his time spent working at the local funeral parlor, it doesn't seem likely that he'll be able to meet a deadline set by Great Aunt Adelaide (ANGELA LANSBURY) where she'll cut off the family's allowance if he can't find a new wife and mother for the children.
With little time to spare, Cedric sets his sights on recent widow Selma Quickly (CELIA IMRIE), but realizes his kids will probably drive her away as well. As luck would have it, that's when Nanny McPhee (EMMA THOMPSON) shows up. Bucktoothed and not particularly becoming, she says she's there from the government and will whip the kids into shape via teaching them five valuable lessons.
Led by Simon, the kids immediately set out to drive her away as well, but with her magic stick and other supernatural powers, it's not long before she puts the kids in their place and teaches them a thing or two about manners, respect and more. But with time running out regarding Adelaide's deadline, it's uncertain whether she'll succeed before it's too late.