The “Seven Deadly Sins”—a group of evil knights who conspired to overthrow the kingdom of Britannia—were said to have been eradicated by the Holy Knights, although some claim that they still live. Ten years later, the Holy Knights have staged a Coup d'état and assassinated the king, becoming the new, tyrannical rulers of the kingdom. Elizabeth, the king's only daughter, sets out on a journey to find the “Seven Deadly Sins,” and to enlist their help in taking back the kingdom.

Straightforward and innocent Hisone Amakasu is a Self-Defense Force rookie stationed at the Air Self-Defense Force's Gifu Base. She was struggling with the fact that she sometimes hurts people unintentionally by her innocent words and decided to join the Air Self-Defense Force, hoping to maintain a certain distance from people. This decision led her to a fateful encounter which profoundly changed her life. It was the "OTF" dragon hidden in the base and it chose Hisone as his pilot. When it soared into the sky with Hisone, her fate as a dragon pilot was decided. It is said that dragons have a key to the future of the world...

Brought together by a mysterious song, a grad student and an engineer lead the fight against an unimaginable force that may spell doom for the world.