Morgan Spurlock spends thirty straight days eating nothing but McDonald's, three meals a day, to test what a fast-food-only diet does to a healthy body. He sets rules before he starts: he must try every item on the menu at least once, and if a cashier asks whether he wants to supersize, he has to say yes. Doctors monitor him throughout, and the results arrive fast: his weight climbs, his cholesterol and liver enzymes spike, and he reports mood swings, headaches, and a plummeting sex drive severe enough that his doctors urge him to stop. Between meals, Spurlock visits schools serving processed lunches, talks to nutritionists and industry lawyers, and traces how fast-food chains market to children through playgrounds, toys, and branding. The film also looks at the lawsuit brought against McDonald's by obese teenagers, using it as a hook for a broader argument about corporate responsibility versus personal choice. It closes with Spurlock's slow physical recovery, a coda that underlines just how much damage thirty days did.