Ursula Holt had spent six months in a lunatic asylum for killing and maiming farm animals while sleepwalking. Like her keepers, she is convinced of her own guilt, as is her kindly uncle, Dr. Matthew Holt. Only her loyal and loving cousin Kate, and the Holts' neighbor, wealthy Alexander Skeffington, stoutly maintain Ursula's innocence. Then, one of Dr. Holt's scalpels is used to cut a young American visitor's throat, and Ursula is immediately suspected. Detective Inspector Jackson is called in to investigate. After a second scalpel murder, Ursula flees in peril of her life. There follows a frantic investigation, both in England and America, when the chilling truth underlying Ursula Holt's somnambulism is finally brought to the light of day.