In Star Wars: The Phantom Menace, Shmi and her son are slaves of the junk dealer Watto. Qui-Gon Jinn attempts to free them both through a wager on the Boonta Eve Classic pod race, but Watto refuses to wager two slaves against Anakin's pod. Consequently, Shmi is left behind on Tatooine. Some time in the next ten years, Watto sells Shmi to the moisture farmer Cliegg Lars, who frees and marries her.
In Star Wars: Attack of the Clones, she is captured and tortured by a tribe of Tusken Raiders, but she remains alive until Anakin sneaks into their camp to find her, at which time she dies in his arms. The turmoil of his mother's death is one of the first events that begins Anakin's descent to the Dark Side.
Many fans suspect that the Sith lord Darth Plagueis impregnated her without her knowledge through his mastery of the Dark Side of the Force and ability to manipulate the midichlorians. His motivation would be revenge, having foreseen his own death at the hands of his apprentice and therefore created Anakin to eventually destroy his murderer.