For generations, fat has been the enemy. We've demonized it as a cumbersome health risk and cast overweight individuals as too gluttonous or lazy to make healthy choices. But scientists are coming to understand that fat is not so simple. In fact, it's a fascinating and dynamic organ -- one whose size has more to do with biological processes than personal choices. Now, NOVA takes you inside the amazing world of fat. Why don't sumo wrestlers don't suffer from the health problems that other obese people do? Why has evolution hardwired us to hang onto fat even when it's unhealthy? And what would happen if you had no fat at all? Through real-life stories of hunter-gatherers, supermodels, and a Biggest Loser contestant, NOVA explores the complex functions of fat and the role it plays in controlling hunger, hormones, and even reproduction.