Bunretsu Tanbara, Gai Tanbara's younger brother, secretly stows away inside Gai's time machine during the mission to investigate the temporal displacement of Dallas and Kujukuri. His unauthorized presence immediately complicates the journey. After arriving 10,000 years in the future, Bunretsu becomes separated from Gai during exploration. He encounters perilous mutant monsters in the desolate wasteland, only to be rescued by an old hermit named Jinba. Bunretsu then remains with Jinba in the desert, serving as his acolyte and aiding in daily survival tasks.

During this time, Bunretsu observes Jinba rigorously training Emiya, molding her into a capable warrior. Jinba later discloses his identity as the exiled king of Kukurit and reveals Emiya's true heritage as Princess Prime Rose, succumbing to mortal wounds immediately after. Bunretsu's behavior swings erratically between cowardice and recklessness; he cowers when confronted by creatures or hostile forces yet impulsively charges into fresh dangers moments after being rescued.

He displays juvenile tendencies, including spying on Emiya while she bathes nude in a forest spring. Later, Bunretsu accompanies Emiya—now acknowledged as Princess Prime Rose—aboard giant butterflies to infiltrate Prince Pirar's palace, joining the battle against Guroman's forces. In the climactic conflict against the entity Sigma-99, Bunretsu supports Gai and Prime Rose, though his contributions remain auxiliary rather than pivotal. His actions consistently weave comic relief with flashes of vulnerability throughout the entire ordeal.