To find out how a real pandemic might spread through Britain, mathematician Hannah Fry and a team of epidemiologists recruit over a thousand volunteers in the small town of Haslemere to install a tracking app on their phones for a week, logging every contact and every meter of movement. The data feeds computer models built to simulate contagion across the whole of the UK, translating one town's ordinary Tuesdays into a national forecast of who catches a hypothetical virus first and who spreads it furthest. Scientists explain why a school run or a commuter train carries more epidemiological weight than most people assume, and why the old assumption that pandemics move at random turns out to be wrong. The film intercuts the Haslemere experiment with the history of past outbreaks and the statistical tools now used to predict the next one. It is a demonstration film as much as an explainer, showing exactly how the data was gathered and what the resulting maps and graphs reveal about the shape a real epidemic would take.