Four suns bring blessings to the world. When one of them vanished, people grieved. Nia, a sky explorer, and her automaton Mano set out to investigate the misty tower where the sun was said to have fallen, but they were caught in a storm and their airship broke down. They decided to repair their airship inside the tower while continuing their search for the sun. "Well then, let's go investigate right away!" However, the inside of the tower was an abnormal space, a complex labyrinth teeming with monsters. Furthermore, ghosts appeared, and Nia was captured. Mary, who controls the ghosts, is somewhat carefree, but it seems she will never tolerate intruders in the tower...