The Family Plot

What would drive a church-going Springfield, Missouri, woman and her daughter to fatally poison two family members with anti-freeze, and attempt to kill a third over a 14-month period? Diane Staudte and daughter Rachel both confessed to the murders of Diane's husband, Mark, and her son, Shaun, and attempting to kill her daughter Sarah, and are both serving life terms in prison. "20/20" takes viewers inside the case, revealing new details of the heinous murder plot. ABC News correspondent Deborah Roberts sits down with Sarah Staudte, the surviving victim, who describes how she discovered that her mother wanted her dead. The hour also features interrogation tapes of Diane and Rachel Staudte describing, in their own words, how and why they did it, as well as interviews with detectives, prosecutors, and friends and family members of the victims.