About three times a day our sky flashes with a powerful pulse of gamma ray bursts (GRB), invisible to human eyes but not to astronomers' instruments. Where these bursts originate, and how they come to have such incredible energies, is a mystery scientists have been trying to solve for three decades. Based on interviews with leading scientists, Flash! provides an insider's account of the scientific challenges involved in unraveling the enigmatic nature of GRBs.