A global nuclear war has turned most of Earth into a contaminated wasteland, with humanity's survivors now fighting over the few uncontaminated food and water supplies still remaining.
Kenshiro, Akira Kamiya is a master of the deadly Hokuto Shinken, traveling with his fiancée Yuria, Yuriko Yamamoto, when they are confronted by a gang led by Shin, Shigeru Chiba, a former ally who proclaims his love for Yuria and challenges Kenshiro with no law to interrupt him.
After defeating Kenshiro in combat, Shin engraves seven wounds on Kenshiro's chest and leaves him for dead, taking Yuria with him.
Kenshiro's eldest brother-in-training Raoh, Kenji Utsumi, who has witnessed the fight, returns to his dojo and discovers his sensei Ryuken, Junji Chiba, meditating. Raoh challenges Ryuken's decision to choose Kenshiro as the Hokuto Shinken successor over him and kills Ryuken, declaring that he will become the ruler of the new world.
A year passes, and Kenshiro roams the wasteland as a protector of the weak. He rescues two young children, Bat Teiyu Ichiryusai and Lin Tomiko Suzuki, from a gang of bandits and allies himself with another Nanto Seiken master named Rei, Kaneto Shiozawa who is searching for his kidnapped sister Airi, Arisa Andô.
Ken learns that Jagi, Chikao Ohtsuka, another former brother-in-training, is behind Airi's kidnapping and is impersonating him to tarnish his name. Ken heads to Jagi's hideout and defeats him, rescuing Airi. Before dying, Jagi reveals that he was the one who persuaded Shin to betray Ken and that he now lives with Yuria in his stronghold, the city of Southern Cross.
Meanwhile, Raoh, the conqueror also known as Ken-oh (King of Fists), has built a vast army and moves toward Southern Cross. In the city, Yuria endures a life of luxury under Shin's rule, but she longs to be reunited with Ken. When she learns that Ken is alive, she tries to slip away, only to be captured again by Raoh.
Kenshiro finally reaches Cassandra, the City of the Wailing Demons, to find the city aflame and Shin, though mortally wounded from his earlier clash with Raoh, still fighting Ken. After Shin falls, Kenshiro finds Raoh's forces parading Yuria in the streets, and Lin and Bat attempt to rescue her from Raoh's dungeon later that night.
A key moment comes when Lin and Bat secure Yuria and bring her a plant grown from a seed Yuria had given Ken. The plant draws Raoh's attention, and Yuria is sentenced to a public execution the next morning. Rei confronts Raoh, but Raoh defeats him, leaving Rei to die.
Kenshiro and Raoh finally unleash their fighting auras in a devastating showdown that destroys much of Cassandra and leaves both fighters gravely injured. With Lin's intervention, Raoh spares Kenshiro's life and promises to postpone their battle. Kenshiro leaves Lin and Bat and continues his search for Yuria, who remains missing after the confrontation.
