Wanted, a respectable woman to care for a motherless child. When William D. Thomas's wife dies in childbirth, he places an advertisement in his local newspaper seeking a nanny for his newborn child.

He is thankful when an experienced nanny arrives at 43 Laurence Street and takes over from his frazzled housekeeper Mrs McHugh. Mrs McHugh confides in her bedridden friend Betty, who has a bird's-eye view of all the happenings on Laurence Street, that the Nanny is not all she seems. Betty begins her own investigation into the mysterious woman.

When the bodies of twin babies are discovered buried in a back garden, by a family who have moved from their tenement home into a country cottage, a police investigation begins. But it is Betty who holds the key to discovering who the Nanny really is ... and the reason she came to 43 Laurence Street.