One day, Minami Matsuo, a high school student who dreams of having the suicide uniform she made worn by a man she recognizes as her ideal watchdog, is attacked by a street demon.
The moment she wonders if her life, which she set out to make her dream come true, will end like this, her consciousness fades away. When she regains consciousness and wakes up, she finds a strange other world full of steam, pipes, and gears that are like fog. The man who appeared before Minami's bewildered eyes, dashing on his steam bike, was Leon Astor, a man known as the "Marquis of Delinquency," and a vampire! Witnessing his strength and power, Minami sees in Leon the image of the hero of a Yankee movie she saw as a child and her ideal gang leader, and decides to have him make and wear a suicide suit!　However, a shocking revelation awaits Minami as she goes ahead with her dream:
A unique love comedy of "delinquent x otherworldly x vampire" unfolds in a vivid steampunk world!