This documentary tells the story of the world's heaviest woman, Renee Williams, a 64-stone, 29-year-old mother of two who, in a desperate bid to save her life, undergoes life-threatening surgery.

Renee, who has realised she wants to be around long enough to see her two daughters grow up, hopes her story will help change the lives of the estimated two million Americans who weigh more than 40 stone. With obesity causing some 400,000 fatalities a year in the US, America is on the verge of eating itself to death.

Renee has been told for years that she was too heavy for a potentially life-saving gastric bypass but now, finally, a pioneering doctor has agreed to operate. Renee invited cameras into her normally private world a week before her surgery. From her bed, in which she has been trapped for the past four years, she talks about her size, her food, her children and her hopes. By going public with her story, Renee hopes that she can and will motivate other super morbidly obese people to seek help too.

Help comes in the form of Dr Nowzaradan at the Renaissance Hospital in Houston, who has performed more than 1,000 successful gastric bypasses and is willing to take on risks that others won't. "She begged people to help her and she went in to doctors' offices and got the door slammed in her face," says patient liaison officer Angie Flores.

Aware the stakes are high, Renee's 13-year-old daughter Mirina hopes that the operation will help her mum regain a normal life. "I would like to see her walking again," says Mirina, "and she'll help take care of us hopefully".

Renee will be the largest person ever to undergo a gastric bypass. The operation is her last chance but it carries a high risk. Either she will be on the way to losing 50 stone, or she'll die.