The novel Ruby is named after the main character in the book, teenager Ruby Landry. Ruby lives with Grandmere Catherine, a Cajun traiteur, in Houma, Lousiana. Ruby's Grandpere Jack has been living in a shack out in the swamp since Ruby was born because he is an alcoholic. Ruby's mother died in childbirth. Ruby happens to be a talented artist despite the fact that she has little money and has not taken advanced lessons. While she and her grandmother sell things at their roadside stand, an art dealer sees her work and buys paintings for his gallery.

Ruby begins seeing Paul Tate, the son of a wealthy family. The relationship is ill-fated. When Grandmere Catherine sees how determined Ruby and Paul are to be together, she reveals to Ruby that Paul is her half-brother. Octavius Tate raped Gabrielle Landry in the swamp, impregnating her. Gladys Tate, wife of Octavius, bought the baby at the demand of Grandpere Jack.

Ruby was born as a result of Gabrielle's affair with Pierre Dumas, a rich man from New Orleans. Grandpere Jack arranged to sell the child to the Dumas family. When Gabrielle gave birth to twins, one baby was given to the Dumas family and the other, she kept.

Ruby breaks up with Paul, telling him that they cannot be together. Ruby is left scared and alone when her Grandmere Catherine dies one night. Paul visits her and devastated Ruby tells him the truth about their family. Grandpere Jack begins making arrangements for Ruby to be married to Buster Trahaw, another alcoholic. As a result, she heads to New Orleans.

Ruby finds her father's house in the Garden District with help from a woman on a bus. Outside she meets Beau Andreas who first believes that she is Giselle, the twin. Ruby explains the situation to Pierre and Daphne when she meets them and they agree that they will take Ruby in. They make up a story that Ruby was kidnapped as a baby, to tell their friends.

Daphne and Giselle are not receptive to Ruby and treat her badly. Giselle is mean and, while conniving, not as intelligent as Ruby. Giselle and Beau are a couple, but Ruby's presence tears them apart. Giselle becomes jealous when Ruby and Beau begin dating. Giselle's new boyfriend is Martin Fowler. As a result of drug intoxication, the two get into a car accident that kills Martin and cripples Giselle.

Ruby creates a piece of artwork for which Beau posed nude. When Daphne finds it in the studio's closet she tries to get Ruby locked away in a mental hospital with Pierre's brother, Jean. She tells the doctor that Ruby is a nymphomaniac. Ruby is able to escape after she meets her uncle. When Ruby tells Pierre what Daphne tried to do, he just wants everybody to be happy. The family, with the exception of Giselle, determines that the twins should go to boarding school in Baton Rouge.