In 1914, in the infamous Paris district called "The Sock," sewer cleaner Chico rescues the timid Diane as she is being beaten by her brutal sister Nana for not entertaining a male customer in Nana's seedy café. While Chico is lamenting his fate and discussing his atheistic beliefs with his friends, Boul and the Sewer Rat, kindly Father Chevillon offers to get Chico a coveted job as a street washer if he will care for Diane and try again to believe in God. Chico agrees, and when Nana's angry customer brings the police to the café, Chico tells them that Diane is his wife so that they will not arrest her. One of the gendarmes threatens to come to Chico's apartment to check on their story, and so Chico is forced to take Diane home with him. As the pair talk, the brash and cheerful Chico tries to impart his optimistic philosophy of life to Diane. After they climb the seven flights of stairs to Chico's airy, light-filled garret, Diane, who is used to the squalid surroundings of The Sock, declares that it is heaven. Chico laughs and sends her to bed, but that night, he lies awake thinking of her, just as she is thinking of him. After Chico leaves for work the next morning, the gendarme visits the apartment and, satisfied that Diane lives there, says he will not be back. Diane is about to leave when Chico returns and tells her they will celebrate his new job with their neighbors, the Gobins. Diane does not have the heart to tell Chico about the gendarme's visit, but after he leaves to get some wine, his eccentric neighbor Aristide chastises Diane for keeping the secret. After he accuses her of taking advantage of Chico and convinces her that her wickedness will destroy any chance he has of achieving greatness, she leaves. Chico is distraught to discover that Diane is gone, and after learning what Aristide told her, he finds her in a bar with a man who is attempting to seduce her. Chico hits the man and takes Diane home, where he tells her that she is fine and good and should remain with him. Time passes as the young people fall in love and Diane overcomes her timidity. Chico proposes to her, but on the day they are to be married, he receives a draft notice. Before he leaves, however, the lovers exchange vows and declare that they are married in the eyes of God. After they promise to think of each other every morning at 11:00, Chico leaves with Gobin. Once Chico has gone, Nana comes to the garret and attempts to take Diane back to the café, but Diane, strengthened by Chico's love, beats her sister until she flees. Four years pass as Chico fights in the trenches and Diane works as a nurse. The couple think of each other every day at 11:00 a.m. and mentally send the message "Chico, Diane, Heaven." After the war ends, Gobin and Aristide, who has become Diane's friend, try to convince Diane that Chico was killed, but she refuses to believe them until Father Chevillon gives her the medal that Chico gave to him when he lay wounded on the battlefield. Diane is heartbroken and renounces her belief in God, but when the clock strikes 11:00, she once again feels Chico's presence. She rushes home and finds Chico, who was only blinded in the explosion that was thought to have killed him. The young lovers embrace and tearfully reaffirm their belief in each other and in God.