In this dark and witty fable, Thompson portrays 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 exceedingly ill-behaved children. The children, led by the oldest boy Simon, have managed to drive away 17 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 vile behavior now leads swiftly and magically to rather startling consequences.