What would happen if your best-friend became your new sister? Mary-Kate and Ashley find out when they swap each other for their best-friends Britany and Melanie! Mary-Kate inherits a baby brother in the exchange and discovers that infants may be cute, but changing diapers isn't , while Ashley discovers that super cool fashion-designer dads are fun, but the high life without your true blue sister to share it with isn't really worth it either.