William Rackham is a failure as a writer, deeply in debt, and husband to an increasingly unstable wife. When his father cuts off his allowance, William seeks solace with a prostitute, Sugar, and discovers that she’s as skilled at conversation as she is at sex. For her own part, Sugar sees William as a way out of Mrs. Castaway’s brothel.

William wants Sugar all to himself and moves her into an apartment. Now involved in his father’s business, he regularly gives her money. A chance meeting at the opera convinces William’s disturbed wife, Agnes, that Sugar is her guardian angel.

Now too busy to visit Sugar’s flat, William moves her into his home as live-in mistress and governess to Sophie. Sugar quickly bonds with the lonely child and learns more about Agnes. Meanwhile, the malevolent Dr. Curlew convinces William to have his wife committed to an asylum.

Sugar denies helping Agnes. Now pregnant, Sugar hopes in vain that William will accept their child, but he has other, more socially respectable plans that don’t include her. Desperate and enraged, Sugar takes matters into her own hands.