On 4 June1913 Emily Wilding Davison stepped into the path of the King's horse at the Derby and was fatally injured.
Astonishingly, the terrible moment was captured on three newsreel cameras. The footage of a well-dressed Edwardian woman being killed by racehorses travelling at 35 miles an hour remains deeply shocking.
But though the act itself was captured in horrific detail, a century on, mystery and argument surround the story behind it: what exactly Emily Davison intended to do on the track that day; and what drove her to take such reckless action in the first place.
Clare Balding and a team of forensic experts have analysed the footage frame by frame, re-examined the evidence and, astonishingly, believe they may have made new discoveries that will change our view of what really happened on that fateful summer day.
Clare uncovers the story of Emily herself and finds out how a middle-class governess from a genteel family became a radical activist.
And she explores the hidden history of the militant wing of the votes for women campaign to which Emily belonged, revealing stories of terrible police brutality, forced feeding of hunger strikers, the early use of surveillance tactics that persist to this day and women prepared to use any means necessary to advance their cause.