In Australia, mangled kangaroo corpses litter a roadside. Researchers point to an extinct breed of marsupial: the Tasmanian tiger. Could a predator come back from the dead to terrorize a town? In Redondo Beach, Calif., millions of sardines turn up dead in a harbor. Fearing pollutants, the community forms a relief effort to clean the contaminated waters, but a world-changing event takes the investigation in a new direction. In Minnesota, what's causing a colony of blue herons to flee their nests?