It is the early 1920's, Russia. Eighteen year-old Kira Argounova (Alida Valli) and her family have just returned to St. Petersburg, and are gradually learning to cope with Russia's sweeping social and ideological changes in the wake of the Communist Revolution. Kira loathes the new order and openly defies those who pressure her to conform. In spite of her family's protests she vows to attend the university with the goal of becoming a structural engineer. Returning home one night through a shady section of the city, Kira encounters a young man, Leo Kovalensky, (Rossano Brazzi), who assumes she is a prostitute. They find a deserted garden where Kira learns that Leo is a fugitive, hiding from the GPU--the secret police. Kira reveals that she is actually a student. Unknown to her family or friends, the pair continue to meet secretly and they soon fall in love. Following a close call with the GPU, the two lovers attempt to escape Russia on a smuggler's ship. They are caught and Leo is taken away. Kira later meets with the arresting officer, Timoshenko, and learns that Leo was wanted because he is the son of a former aristocrat executed for fighting against the Communists. With no specific charges against Leo, the sympathetic Timoshenko is able to secure Leo's release. No longer a fugitive, Leo moves into a tiny apartment and begins classes at the University. Kira, defying family and convention, moves in with him. Kira meets Andrei Taganov (Fosco Giachetti), a top GPU official and idealistic revolutionary, and despite the clash of ideologies, a mutual respect forms between the two. Leo soon discovers that, because of his aristocratic family background, no one will hire him -- except for hard, manual labor. Each night he must also put long hours into his studies. His health deteriorates. Kira and Leo are devastated when, suddenly, in a political purge, both are expelled from the University. Leo's will to go on begins to fade. He develops tuberculosis and needs to be sent to a sanatorium for treatment. Kira tries desperately to get him accepted in a state-run institution, but officials refuse to help anyone with Leo's background. Their last hope is an expensive private facility. Kira must somehow find the money or Leo will die. Late one night, Kira pays a visit to Andrei. In an agonized confession, Andrei reveals his love for Kira, unaware of her relationship with Leo. Seeing a way to save Leo, Kira pretends to return Andrei's love. Leo leaves for the sanatorium, never suspecting that Kira is using Andrei's generous gifts to pay for the treatment.