After Char's rebellion come thirty years of peace and prosperity. Side 4, destroyed during the One Year War, is renamed the Frontier Side and construction of new colonies begins.
At the same time, a new force is preparing to challenge the Federation. Recruited, equipped and trained in secret, the Crossbone Vanguard are an elite private army in the employ of the aristocratic Ronah family. They are dedicated to the Ronah family's "cosmo nobility principle", which holds that humanity should be led by the best, the wisest, the most courageous - not by petty bureaucrats and corrupt politicians. Now they plan to establish a perfect society based on these ideals, and they've chosen the Frontier Side to become their "Cosmo Babylonia." In UC 0123, the Crossbone Vanguard make their move.