Umibouzu vs. Utsuro and Kagura vs. Kamui. Though along the way, the father and son reveal the figure who has a connection to all four of them, the deceased wife and mother of three of them, Kouka.
Two people sit in a room on a spaceship: an older man sitting on the windowsill looking out the window watching the stars and the other, a young girl sitting near a table drinking tea. The man admits that the title "strongest in the universe" was a hollow one born from people who have never seen the vastness of space and are aware of only a tiny part of it. The girl, amused at his modesty, responds that his title enabled him to become her tutor, thanks his friendship with her father, the captain of the space pirates Chidori. The girl, Mutsu, decides to refuse the tutor offer for the man, Kanko (the young Umibouzu), believing she wasn't worthy of it. Instead, she asks if he could tell her tales of his travels and exploits all over the universe.
In the present, Umibouzu and Utsuro stare each other down. The immortal believed that their confrontation was inevitable, a result of all creatures' desire for conflict for superiority. Especially the case between two of the strongest beings in the universe. Umibouzu disputes his title, telling the Tendoushuu member that he only cares about protecting what only he cares about. Utsuro counters that the hunter was more than that, but he will still be susceptible of defeat, which he believes will be given by him, the person beyond life. Utsuro attacks and the sound of their fight is heard by Mutsu on the lower cliffs. She worries for her teacher but Sakamoto tells her to hurry.
In the past on the Chidori ship, Mutsu asks Umibouzu on who he believes deserves the title of universe's strongest. Umibouzu offers his rival Hosen but only so far of strongest of the Yato. During much of his travels, he had seen creatures and natural disasters that are far beyond the concept of "strong". Mutsu then reveals the reason Umibouzu earned the title: centuries ago, the Yato's home planet was attacked by the Allied Planets during the Great War and was destroyed, nearly wiping out the Yato and the native species. The planet's, Kouan, Altana was released, giving birth to an ecosystem of mutant creatures, including a dangerous planetary parasite. The parasite named Orochi feeds on Altana, grows up to 108 heads, and was extremely territorial, resulting in any attempts to recolonize the planet futile. Yet a few years before the "present", one man entered Kouan and took back the planet on his own, Umibouzu. The hunter corrects her though, he had lost a bet to a friend and the punishment was to visit the dangerous planet to pick up girls. He then describes how when he encountered them, he had an erection, his primal instincts telling him that death is near so he should start procreating. He had realized that despite going against them, his power meant little, as if fighting the planet itself. But he forcefully suppressed his instincts by squeezing his testicles and fought on... to finally have a chance to hit on the woman. His statement confuses Mutsu as she thought he was talking about the parasite. At the same time in the present, Umibouzu's fight with Utsuro is an echo of his "fight" in Kouan, except mentally telling his instincts that he already left great seeds so let him fight for his family. The two men punch each others' fists, leading to Utsuro losing his sword arm and the hunter witnessing his regeneration. Umibouzu concludes that Utsuro was a being of Altana just like another person he knew. Then it's revealed that the "them" and the person were the same, the woman he found on the planet.
In Kouan, Umibouzu was immediately attacked by one of the Orochi's heads when a woman arrived and knocks it out. The young hunter immediately gets an erection upon seeing her strength and beauty and states his surprise at seeing someone still living on this dead world. She retorts that there are still living beings that are surviving here including her, then chastises him for that line of thinking. She walks away, telling him to leave as his presence has awakened the Orochi and she must put them to sleep. While she fights off the heads, Umibouzu concludes that she was the true master of the planet, not the parasite and runs behind her, also fighting off some of the heads. His poor attempts at trying to woo her results in her both beating him up and ignoring him; three days later, the Orochi retreats and Kanko was able to suppress his horniness. He introduces himself and apologies for causing a ruckus, stating that he will be a proper guest next time he returns. The woman instead responds that he can return whenever he wished, plus the Orochi was actually glad to have guests since it's been centuries since someone visited. She introduces herself as Kouka.
Afterwords, Kanko started to visited Kouan frequently as well as noticing that the Orochi no longer "attacks" him, most likely accepting him. In her home, Kanko wonders why Kouka live here as there was noting to eat or drink except poison. She answers that she can't live anywhere else but here; the surviving Yato scattered to other planets but a few remained to die with their home, some died out, some adapted. She was a descendant of the Yato who had adapted to the now harsh planet, the only living Yato. Kanko insisted that she was lonely being by herself yet she refused to leave, though she counters that she liked to do whatever she wanted by being alone. Umibouzu decides for the next few days to tell her tales of his adventures on different planets but he wasn't sure of her feelings about it due to her constant bored expressions. Until she interrupts his tale to insists on physically seeing a planet herself. He takes her on his ship and they look at a planet through the window; he notes that despite her usual expression she was fascinated at the planet. He talks about the planet Earth and asks if she wants to visit someday, along being his "partner"; to his surprise, she agrees. Also to his surprise, Kouka sees him off as he leave the planet.
The next time he returns, the woman disappears and he spends twelve days searching for her to little avail. When he sees an Orochi as a sentinel, he finds her within a small forest near it. She admits that she hid because of her confliction between staying in Kouan as her duty and wanting to leave with the man she fell in love with. He also confesses that he also loves her and will stay by her side forever. The female Yato chooses to leave the planet but feels as if she's abandoning it; instead, Kanko insists that the planet will thank her. The two leave for the last time, the Orochi quietly seeing them off. After hearing the story, Mutsu was amused of the truth behind the legend and Umibouzu admits that the strongest beings in the universe were... wives. Mutsu gets a good chuckle out of this and was happy to hear about stories of friendship, parenthood and happiness. Umibouzu agrees, remembering seeing his newborn daughter with his son Kamui. But the moment of happiness sours when Kouka begins to cough out blood and Umibouzu realizes the truth in her statement that she cannot live anywhere else but Kouan. He admits that he was niether a hero nor a father.
In the present, Kamui and Kagura survive their fall, and Kamui declares to his younger sister that Umibouzu was the murderer who killed their mother.