When her father is murdered, Maria (Valentina Cortese) moves to London with her young sister. They live with Anselmo (Charles Goldner), a family friend, and resume a normal life. After a few years, Maria and her sister, Nora (Audrey Hepburn), visit Paris with Anselmo. There, Maria meets an old love, Louis (Serge Reggiani), who has become a radical and currently plots to assassinate a world leader. Louis uses Maria to enter London and then convinces her to begrudgingly join their terrorist group.