In the detective business, everyone knows the name Keiichirou Nagumo. Back in high school, he was called a genius for cracking tough cases that even the police could not solve. But those days are long gone—or so the rumors say. However, one day, an unexpected visitor barges into his office: Mashiro, an energetic high school student who demands to be hired as his new assistant.

Although Keiichirou tries to brush her off, Mashiro is persistent. She is determined to become a detective, and to prove her worth, she begins by teaching him about the latest technology and offers a fresh perspective on his business. Thus, Keiichirou takes Mashiro on as his assistant and embarks on a brand new chapter in his career—one where he finds himself constantly surprised by the modern generation of detectives.