450 years prior to the present, an extraterrestrial force called Efy Dolgh invades Earth; the people of Sengoku era Japan dub their mecha and robotic drones as oni. The Washiba Clan falls victim to them, but clan heir Yukihime and her samurai retainer Kennosuke Tokisada Ouma fight back with a stolen Efy Dolgh mecha, which they dub the "Black Relic" (Kuromukuro?). They succeed in vanquishing their foes, but when the Black Relic is caught in an enemy explosion, Yukihime disappears. 60 years ago, the Black Relic is unearthed during the construction of the Kurobe Dam. In the present day, the United Nations is researching it when the Efy Dolgh return. Yukina Shirahane, daughter of the UN facility director, accidentally triggers a cube component of Kuromukuro, releasing Kennosuke from cryostasis. Aided by Yukina's family and the UN in adjusting to life in the 21st century, he stands against Efy Dolgh's invasion of Earth just as he did in his home era. As the Black Relic is the most effective weapon the UN has against Efy Dolgh technology, Kennosuke becomes one of their most valuable assets. Due to an accident, Yukina is biometrically synchronized with the Black Relic, and as it requires two pilots, Yukina thus becomes Ouma's combat partner.