In this dark and witty fable, Emma Thompson portrays a person of unsettling appearance and magical powers who enters the household of recently widowed Mr. Brown and attempts to tame his seven exceedingly illbehaved children. The children have managed to drive away 17 previous nannies but as Nanny McPhee takes control, they begin to notice that their vile behavior now leads swiftly and magically to rather startling consequences.

Nanny McPhee arrives to help a harried young mother who is trying to run the family farm while her husband is away at war, though she uses her magic to teach the woman's children and their two spoiled cousins five new lessons.

Mr. Cedric Brown has just lost his wife and is now left with his seven children who misbehave so much that all the nannies have run away. Now he is told by a mysterious voice that he should get Nanny McPhee who is a magical woman with special powers.