Players assume the role of either a human male character named Vent or his female counterpart, Aile, who are employees of the Girouette Express, a courier service that takes jobs transporting anything to anywhere. On a job accompanying their boss, Girouette, to transport a package containing Model X and Model Z Biometals, they are attacked by a mysterious group of Mavericks who seeks to take the material for themselves. During the scuffle, the Model X Biometal is absorbed into the main character, causing a transformation into Model X form while the Model Z Biometal fuses with Girouette. Facing innumerable enemies, Girouette lends his newfound power to his employee and the two substances combine to form Model ZX. With powerful abilities at their disposal, they must discover who the menacing enemies are and what they are ultimately after.