Enigmatic Nanny McPhee (Emma Thompson) arrives on the doorstep of a harried mother, Isabel Green (Maggie Gyllenhaal), who is trying to raise her kids and tend to the family farm while her husband is away at war. Isabel has her hands full with not only her own three, but also with her sister's spoiled pair. With no time to lose, McPhee uses magic to instill in the children five important lessons.