Annie Parker (Samantha Morton) finds out she has breast cancer—a devastating blow to a woman who has already lost her mother and sister to the same disease. Meanwhile UC Berkeley geneticist Mary-Claire King (Helen Hunt) is convinced there is a link between DNA and cancer, even if few in her profession believe that some cancers may be hereditary. Based on true events, the film chronicles how these remarkable women work toward one of the most important genetic discoveries of the 20th century.