Five hundred years ago, angered by humanity's sins and arrogance, God recalled his Angels from Earth and the surface was consumed by Chaos. The last person who still prayed to God, the first Saint Irene, asked for his forgiveness. God listened to her prayers and lifted the city and land around her to save humanity from extinction. As the last bastion of humanity, the city of Novus Aether floats in the sky above the inhospitable surface, powered by the daily prayers of the Saint. More than a decade ago, due to the interruption of the prayer, the "Gran Forte" occurred, causing part of the city to fall to the surface and splitting the lower city in two.

In time, the lowest layer became the dwelling of all kinds of low-lives, people who lost much of their former lives during the "Gran Forte," becoming known as the "Prison." However, a contagious disease that causes people to grow wings has spread throughout the city. A special unit has been formed to deal with these people swiftly.

In this "Prison" resides Caim Astraea, a freelance ex-assassin who does odd jobs for his friends for money, refusing jobs that require him to kill people. During one request, Caim finds a girl who contracts the wing-disease, enveloped in a light that reminds Caim of the "Gran Forte" years ago...