In 1918, an influenza strain moves through a world already exhausted by the First World War and kills an estimated 40 million people in about a year, more than the war itself. This film follows the virologists and epidemiologists who, eighty-five years on, are still trying to answer two questions: where the strain came from and what made it so lethal. It traces their methods, from tracking outbreak patterns across military camps and civilian populations to searching for genetic material from victims buried decades earlier, work aimed at reconstructing the virus itself rather than just its death toll. The film treats the 1918 pandemic as an open case rather than settled history, framing the scientific hunt as one with direct stakes for recognizing the next dangerous flu strain before it spreads as fast as this one did. Sources here are thin on specific scenes and named researchers, so the account stays close to the pandemic's scale and the ongoing scientific effort it provoked.