The whole world was devastated by the nuclear war. Under the strong rule of the World Empire, people are all suppressed and deprived of their own lives. A resistance team secretly keeps fighting to break the suppression while everyone is forbidden to possess any force of arms. The team arms themselves with "antique guns" that were left as works of art, and the incarnations of those antique guns are called "Musketeers" appear and join the team, as if they responded to the soul of the resistance. In their battles with the incarnations of contemporary guns who represent and are dispatched from the Empire, the "Musketeers" turn their "Absolute Noble" mode to give a powerful boost to themselves, and bring hope to the world.

Source: ANN

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Side story: Senjuushi: Kijuushi-tachi no Happy Birthday!