The National Football League, a multibillion-dollar commercial juggernaut, presides over America's indisputable national pastime. But the NFL is under assault as thousands of former players and a host of scientists claim the league has covered up how football inflicted long-term brain injuries on many players. In this special investigation, FRONTLINE and prize-winning journalists Steve Fainaru and Mark Fainaru-Wada of ESPN reveal the hidden story of the NFL and brain injuries, drawn from their forthcoming book League of Denial: The NFL, Concussions and the Battle for Truth (Crown Archetype, October 2013). What did the NFL know and when did it know it? What's the truth about the risks to players? What can be done? The FRONTLINE investigation details how, for years, the league denied and worked to refute scientific evidence that the violent collisions at the heart of the game are linked to an alarming incidence of early onset dementia, catastrophic brain damage, death, and other devastating consequences for some of football's all-time greats.