D.D.D.

The kingdom of Garlonia has expanded its military influence through the Black Knight Squadron, leading to increased prosperity. Julius, who lives in the kingdom's slums with his twin brother Michael, aspires to become a strong warrior like his father and escape their difficult life. He sees an opportunity when a special entrance exam is held for the Black Knight Squadron. During the test, Julius discovers that the figures he admired are not heroes but demonic entities known as Devil Devices. He also experiences an unexpected change when a demon takes control of his left hand.