Emma Thompson plays the title-role in her own-scripted screenplay, a dark and witty fable about a person of unsettling appearance and magical powers who enters the household of the recently widowed Mr. Brown (Firth) and attempts to tame his seven badly-behaved children. The children have managed to drive away seventeen previous nannies and are certain that they will have no trouble with this one. But as Nanny McPhee takes control, they begin to notice that their bad behavior now leads swiftly and magically to rather startling consequences.