On Mother's Day, 1946, in San Francisco, Ziggy Brennan thinks back on the events which have brought her to this day: In 1938, young Ziggy's mother Natalie makes her pretend they are sisters and call her Nat, and teaches her how to trick men into giving her money. By the time she is a young adult, Ziggy's ambition and wiles attract a fellow con artist, Denny Reagan. Although Denny's pious mother worries for his soul and donates all the money he gives her to the church, he continues to work scams, and convinces Ziggy to help him. One evening at a nightclub, Ziggy meets a simple, handsome soldier, Mart Neilson, whose watch Denny admires. Ziggy steals it, but Denny later sees the watch's inscription from Mart's mother, and demands Ziggy return it. When she finds Mart, he insists that she date him, and although she admits that she stole the watch and brings him to spy on her mother as she cons a man into giving her money, Mart continues to love Ziggy. Days later, they marry and Ziggy becomes pregnant, but Mart is soon killed in combat. Around the same time, Denny is arrested and sent to the penitentiary. Soon after, Nat decides to marry and tells Ziggy to find her own apartment. When the baby, Martha, is born, the neighbors help Ziggy learn to care for her. Ziggy loves Martha, but continues to remember her mother's scornful prediction that she will be a bad mother who will only want to carouse. When a teenager, Olivette, offers to babysit, Ziggy hires her and begins to go out nightly, unaware that Olivette is leaving the baby alone as soon as she falls asleep. When Martha is eight months old, Denny is released from prison and visits Ziggy. He has reformed and wants her to do the same, and together they care for Martha during the day and go out each night. One night, Olivette leaves with her boyfriend and Martha falls out of her crib and almost chokes to death on the sheets. After Ziggy's landlady sends Martha to the juvenile authorities, Ziggy attacks her and is arrested. During her trial, Ziggy's neighbors accuse her of being neglectful, and the court places Martha in an orphanage. Denny shuns Ziggy for being a bad mother, and she soon becomes despondent. After Ziggy spurns Denny's mother's offer of help, she runs to the church. There she finds an abandoned child on a pew and takes him home. Months later, Denny is about to leave for Florida when he passes by Golden Gate Park and sees Ziggy with the baby, Denny, Jr. He checks up on her and, upon discovering that she is a model mother, begins a relationship with her again. Soon, Ziggy returns to the court, where the judge admonishes her for taking in a child but recognizes that she has become a devoted mother and returns Martha to her care. After Denny, Ziggy, Martha and Denny, Jr. leave the courthouse together, he asks her to marry him.