In 21XX, the dark skies of Second Tokyo were shrouded in acidic clouds. In this city, evil conspiracies spread rapidly, and heinous crimes threatened the lives of its citizens. Amid the chaos, Tsukumori Mio, a young and beautiful female police chief at Tsukuma CITY Police Station, was known as the heroic vigilante Dark—a masked warrior clad in a special reinforced suit who fought evil while upholding the law, dedicating her nights to justice. One day, a small traveling circus arrived in town. A few days later, Mio received an anonymous tip accusing the circus troupe of being behind a series of female abductions. Frustrated with her subordinates' incompetence, Mio decided to infiltrate the circus tent late at night. But this was a trap designed to capture the heroine. Though Dark had previously defeated dangerous gangs, corrupt mega-corporations, and evil syndicates, this time she let her guard down for just an instant against mere street thugs, resulting in her true identity being exposed. Using her secret identity as leverage, they forced her into humiliating acts, crushing her pride completely.