Aria H. Kanzaki is a tiny, silver-haired powerhouse. She is a Quadra-rank Butei from London, armed with twin Colts and twin swords. She is also the great-granddaughter of Sherlock Holmes, though she lacks his powers of deduction—she makes up for it with pure, aggressive firepower.

In a world where international crime has outpaced traditional police, the Tokyo Butei High School trains "Butei"—armed detectives-for-hire who specialize in combat and investigation.

Kinji Tohyama is a cynical student at Butei High. He wants nothing more than a normal, boring life. He actively hides a secret genetic trait: . When Kinji becomes sexually or emotionally stimulated, his brain functions at thirty times the normal rate, turning him into an unstoppable, hyper-logical combat genius. To Kinji, this state is an embarrassing curse he tries to suppress.