It was originally announced in 2003 as a collaboration between Studio Gainax, Takara, and Brocolli, and was planned to air as a television anime movie in 2005. However, it was put on indefinite hiatus after the project's staff members left one after another. One decade later, the project was announced to have revived, with Bushiroad president, Takaaki Kidani, credited as the original concept creator and executive producer, and the late novelist, Sunao Yoshida, still credited as the original story creator. The main mecha design is handled by nitro+, who previously designed Demonbane, while the animation is produced by Kinema Citrus and Orangenote .
In a time when the world is in the pits of despair after being ravaged by a poison called "Shinobi" that came to Earth via an asteroid, the people ran away to a place called "Kairiku," that used to be at the bottom of the ocean, so as to try and protect themselves against the effects of the poison. In such a world, there was but a single light of hope: the "Holy Weapon Giga Road" and the legendary warrior "Yagyu" are needed in order to counter the effects of the poison.
In order to save the world, Ame, princess of a fallen nation, stands up.
Neppu Kairiku Bushi Road provides examples of the following:
- A God Am I: Tsukiyomi is shown to have gain delusions of godhood after giving himself Shinobi cells.
- After the End: The world the story takes place has essentially become a wasteland after being ravaged by the Shinobi, with the only civilizations still standing being completely walled off from the rest of the world.
- Big Bad: Tsukiyomi, Ame's tutor, is revealed to be the main antagonist of the story, having researched the Shinobi cells, integrated them into his own body, and planning on using them to create a new world.
- Hate Plague: The Shinobi are born from human hatred.
- Humongous Mecha: The Shells used by Ame and Sanda are on the smaller side (only about 3 meters tall), but the Giga Road is shown to be a much bigger robot.
- Lost Technology: The Holy Weapon Giga Road is a giant robot that was sealed away long ago.
- Mystical White Hair: Suou's white hair is noted as proof that he's a Yagyu, humanoid weapons created to combat the Shinobi.
- Nice Job Breaking It, Hero: The Holy Weapon was sealing away most of the power of the Jet Matrix, which was the source of the Shinobi. When Ame starts it up, Tsukiyomi can use its full power to destroy the world.
- Super-Soldier: The yagyuu, a race of superhumans created to combat the Shinobi.