A young man starts off on a quest for happiness and is told to head south by a wise astrologer. He later arrives at an ancient castle where two mysterious women live, Tsuki and Yuki. Both women fall in love with the young man. Meanwhile, the young man must face his destiny.

Middle class Sandra, engaged to be married and living in Turin, searches for her missing sister that is estranged from her family. For this, she enters the morally degraded back allies of the port city of Genoa and is exposed to the world of sex trafficking.

The 19-year-old Elena, who is as pretty as she is bright, works as a rice girl for the landlord Guerrini. Having become aware of her name, he finds out that she is his illegitimate daughter of whom he knew nothing. He tries to secretly help Elena, who has fallen in love with the car mechanic Gianni. But that only creates mistrust. When Guerrini's worthless nephew harasses the girl, the situation is dramatically worse.

Edouard Binet, an aimless Frenchman, has been travelling in North Africa for many years, and is sailing to Belgium. En route, he meets Sylvie Baron. He introduces her to Nemrod Lobetoum, a rich Egyptian carrying valuable jewelry, and Sylvie and Nemrod become friends. Their friendship escalates to love, which makes Edouard jealous. Days later, Edouard arrives at a rooming house owned by Mme. Louise Baron, Sylvie's mother, wearing blood-stained clothes. It appears that Nemrod was killed on a train after he arrived in France, but Edouard denies any knowledge of what happened. Sylvie suspects that Edouard is responsible for Nemrod's death, but by now her mother has become Edouard's ally.

Two trains collide: who said that rail was a safe mode of transport? A young Pole girl becomes an orphan and a semi-whore, and Mara (Silvanna Pampanini) would like to adopt her. Alas she lives a life of sin and she and the nuns who take care of the girlie are worlds apart. So she decides she would start over, leaving her pimp (Franco Fabrizi) behind and trying to get a job, which is no easy task. No sooner has she found work in a factory than her former lover (who gave her a child who died when she was young: hence her longing to have another one) turns up. He is played by Marcello Mastroianni who was supporting at the time. Of course he thinks that the new child is his own and as his wife cannot conceive...
This "slave to sin" (check the title) suffers, suffers and suffers; every time she seems to work it out, every time something happens. The movie is a long flashback told by an old woman who stole a medallion in the hotel where she works as a linen maid.