Long ago, the gate to the world of darkness was sealed. Now evil warriors, known as Remnants, are trying to reopen the gate and let demons take over the world. However, at Tobira High School, there exist guardians of the gate who must risk their lives to prevent the gate from being opened.

A short spin-off anime released on "Kouya no Kotobuki Hikoutai: Oozora no Takeoff Girls!" game app and YouTube.

Endless sky and infinite space, the cosmos isn’t some paradise waiting to be found, it is full of darkness and danger, death and terror of the great unknown. However, for that glittering swath of blue, we have no fear! For the longevity of men, we have no regrets! Nothing shall stand in our way because it is finally our time, our age! The age of cosmos exploration!

In the 21st century, mankind lives on two worlds. Following the collision of an ice asteroid, massive terraforming has made Venus a planet now capable of supporting life. Colonists from Earth tamed the hostile world and have thrived for four generations. But they also brought the darker side of humanity. Venus is about to get hostile again. Hiro Seno, a hotshot motorcycle jockey, witnesses the first strike against his country Aphrodia, by the rival nation of Ishtar. Huge battletanks and warplanes quickly lay waste to the city. The Aphrodian army is quick to mobilize and retaliate... and despite his opposition to warfare, Hiro finds himself fighting for his life on the front lines.