Daiichi follows Hinomaru Katsuo, a nuclear engineer at the Fukushima Daiichi Nuclear Power Plant in Japan, and his supervisor Keiinichi, in the aftermath of a catastrophic earthquake and tsunami on March 11, 2011 that wipes out the plant's power supply, leaving no other way for workers to monitor the deteriorating condition of the plant but by walking the powerless, damaged and contaminated buildings and completing tasks by hand.