Drowning Ruth by Christina Schwarz gradually reveals how Ruth Sapphira Neumann, as she always believes, drowns at age three along with her mother Mathilda but is revived. Raised by Aunt Amanda, Ruth learns that her best friend is her half-sister, born the night of the tragedy, and that helps keep her from marrying the son of their common father.

Ruth Sapphira Neumann is a quarrelsome three-year-old in an unstable household, her mother Mathilda having drowned mysteriously and her father Carl serving in the Army in Europe. Ruth retains memories of also drowning, something that Aunt Amanda strenuously (but falsely) denies. Amanda is determined to be Ruth's mother, which adds strain to her mental instability. Amanda carries the guilt of having borne an illegitimate daughter and given her away on the night that her sister drowns.

Amanda eventually allows Ruth to start school. An outcast, Ruth is gradually drawn out of her shell by the most popular girl in school, Imogene Lindgren, who turns out to be Amanda's abandoned baby and, thus, Ruth's half-sister. As the girls mature they attend business school together and talk about going to Chicago to work in advertising. They also attend dances, and fall for the same wealthy young man, Arthur Owens.

Arthur is the son of the married man who long ago impregnates Amanda during a tryst. Amanda is determined to prevent an incestuous marriage. Earlier, Carl suspects that Clement J. Owens, who finds Mathilda's body, is responsible for impregnating her and abating her death. Carl stalks Clement and barely refrains himself from committing murder. Amanda takes Clement out into the lake to tell him the truth and abandons him there, apparently not recalling that he has a heart condition. Amanda and Ruth concoct a letter that succeeds in breaking up Arthur and Imogene. Imogene goes to Chicago to recover from the shock and Ruth settles in to care for Amanda, who has so cared for her.

The mystery of Mathilda's drowning is revealed only in the final paragraphs: having saved Ruth, who also falls through the ice, Mathilda sacrifices her life to prevent Amanda's drowning. Amanda has put down newborn Imogene and is stretched out on the cracking ice, trying to pull Mathilda up. Mathilda bites her sister's thumb to make her let go, leaving a prominent scar. Giving Imogene to a good mother, Amanda starts life over with Ruth.