In a Japan altered by the Amanto alien invasion, the once-respected samurai face degradation and oppression. Determined to restore samurai honor, Kotarou Katsura of the Anti-Foreigner Faction plans to confront the Amanto with force, utilizing a colossal mecha in an assault on the Shinsengumi. Meanwhile, Gintoki Sakata and his companions from the Yorozuya are on a leisurely cherry blossom viewing excursion, only to clash with the Shinsengumi who accuse Gintoki of usurping their spot. What begins as a peaceful outing escalates into a heated conflict resolved through a rock-paper-scissors showdown with intense consequences.