In the near future, planet Earth is permanently altered following the sudden—and tumultuous—arrival of seven unique alien races. In the boom-town of Defiance, the newly-formed civilization of humans and aliens must learn to co-exist peacefully.
Following the events of the season two cliffhanger finale, season three of Defiance opens on the eve of an impending military siege upon the town, which has fallen on hard times following the collapse of its lucrative mines.