In 1930, Maria Brentano (Valentina Cortese) and her younger sister Nora (Audrey Hepburn) flee to London as their father is about to be executed by his country's dictator. Seven years later, Maria unexpectedly meets Louis (Serge Reggiani), her childhood sweetheart, who is engaged in a plot to assassinate the dictator. Maria is persuaded to play an active part in the plan, but it all goes horribly wrong when the bomb they plant kills an innocent waitress, causing Maria much distress.