On May 20, 2013, a ferocious F5 tornado more than a mile wide tore through Moore, Oklahoma, causing 24 deaths and obliterating entire neighborhoods. It was the third time an exceptionally violent tornado had struck the city in 14 years. Yet predicting when and where such killer storms will hit still poses a huge challenge. Why was 2011 -- the worst ever recorded tornado season that left 158 dead in Joplin, Missouri -- followed by the quietest ever year of activity prior to the Moore disaster? Can improved radar and warning technology explain why fewer died in Moore than in Joplin? And will tornadoes get worse as Earth's climate heats up? In this NOVA special, meet scientists in the front ranks of the quest to understand extreme weather events. Also meet storm survivors whose lives have been upended, and learn how we can protect ourselves and our communities for the uncertain future.