London, 1665: bubonic plague sweeps through the city and kills roughly one in three residents before it burns itself out. The film reconstructs the outbreak street by street, tracing how fleas carried by black rats spread infection through crowded parishes, how the wealthy fled to the countryside while the poor were shut into their houses with red crosses painted on the doors, and how mass graves filled faster than parish clerks could record the dead. Diary accounts from the period, including Samuel Pepys's own record of empty streets and tolling bells, anchor the narration in eyewitness detail. Physicians in beaked masks, quack cures, and the Lord Mayor's futile quarantine orders all appear as the film lays out what seventeenth-century London actually knew, and didn't know, about contagion. It closes with the plague's abrupt end the following year, coinciding with the Great Fire that razed much of the city. The film treats the outbreak as both a medical event and a portrait of a society under total strain.