Unit 731 operates as a secret research arm of the Imperial Japanese Army, stationed in Japanese-occupied Manchuria during the Second Sino-Japanese War and World War II, from 1937 to 1945. Under the cover of epidemic prevention and water purification work, its scientists develop biological weapons and test them on prisoners, using human subjects in place of laboratory animals. The film lays out what the unit did inside that base: the research into plague, anthrax, and other pathogens, and the human cost behind the official record of the war's death toll, which the film argues badly understates what happened there. It sits alongside the atomic bombings as another mass-casualty story from the same war, but one built on secrecy and experimentation rather than a single strike. The sources here are limited, so the film is presented mainly as a historical account of the unit's existence and purpose rather than a detailed, scene-by-scene record of individual experiments or survivor testimony.