Gai Tanbara, an agent of the Time Patrol, is charged with investigating the temporal displacement of Kujukuri City and Dallas, sent 10,000 years into the future by the powerful entity Bazusu or Death Mask. His core mission is to halt Bazusu's manipulation of these cities into waging perpetual warfare for its entertainment.

His time-travel expedition unexpectedly includes his stowaway younger brother, Bunretsu Tanbara. Arriving in this future, they encounter a desolate wasteland ruled by warring factions, slave labor camps, and mythical threats like stone-transforming dragons and giant insects. Gai is swiftly captured and forced into labor at a camp constructing colossal demonic statues under oppressive rulers such as Guroman and the tyrant Pirar.

During his captivity, Gai allies with Emiya, a warrior princess fighting to reclaim her homeland. This partnership becomes central as they organize and lead a rebellion among the enslaved populations against their oppressors. Gai adapts his investigative mission into one of active resistance, strategizing with Emiya and fellow rebels to dismantle the slave infrastructure and confront Bazusu/Death Mask directly, transitioning from temporal investigator to battlefield leader committed to restoring justice across timelines.

His protective instincts towards Bunretsu and mentorship roles with figures like sword master Jimba further define his relationships, reinforcing his role as a unifying force among disparate rebel groups.