Michael Mosley turns himself into the experiment, submitting to blood draws, MRI scans, and a battery of medical tests before trying out different approaches to calorie restriction. He meets scientists studying the biology of fasting, including researchers tracking IGF-1 levels and others running long-term calorie-restriction studies on mice and on people who have voluntarily cut their intake for decades. Mosley tries a five-day water-only fast under medical supervision and then settles on an intermittent approach, eating normally five days a week and cutting calories sharply on the other two. Interviews with gerontologists and nutrition researchers lay out the evidence linking fasting to changes in blood sugar, cholesterol, and markers associated with aging and disease risk. The film stays grounded in what the data actually shows rather than promising a miracle, and Mosley's own test results, tracked before and after his experiment, give the science a personal stake. It is a first-person account of a diet built on timing rather than restriction of food type.