Set in a dark, ruined metropolis amidst the post-apocalyptic wasteland, the story takes place 20 years after the end of the Great War. In this collapsed civilization, the assassination organization "Borderline" operates on the fringe of order, upholding their own brand of justice under the creed: "Execute all who deserve it."

The young protagonist July rises from desperate circumstances and grows through hardship. After joining Borderline, he gradually forms a deep and irreplaceable emotional bond with his cold and aloof partner, June. As long-buried truths are slowly uncovered, they finally see through February's mask of hypocrisy and uncover the cruel secrets that have been hidden for so long.