The film opens just after world-famous opera singer Alice Alquist has been murdered. The perpetrator bolted, without the jewels he sought, after being interrupted by a child—Paula (Ingrid Bergman), Alice's niece, who was raised by her aunt following her mother's death.
Paula is sent to Italy so that she can train to be an opera star with the same teacher who once trained Alice. She studies with him for years, all the while trying to forget that terrible night at Number 9 Thornton Square in London.
Paula meets Gregory Anton (Charles Boyer) and soon falls in love with him. She eventually ends her long tutelage to marry him. He persuades her that they should live in the long-vacant London townhouse her aunt bequeathed her and, to help calm her anxieties, suggests they store all of Alice's furnishings in the attic. Before they do, Paula discovers a letter addressed to her aunt by a man named Sergius Bauer, dated only two days before the murder, tucked away in a music book. Gregory's reaction is swift and violent, but he quickly composes himself, explaining his outburst as one of frustration at the bad memories his bride is experiencing.