At only 17 years of age, Rinna Milford is the youngest police inspector at the London Police Department. Solving every crime she comes across, Rinna quickly gains a reputation as a composed, expert detective whose sole focus is the task at hand. However, her image differs significantly from her thoughts, as Rinna wants to be perceived as more feminine. Nonetheless, her coworkers' expectations make it hard for her to behave like a girl her age.

After arriving at a crime scene, Rinna encounters Meishi Leland, a clumsy and playful private detective. Suddenly, an unfamiliar feeling arises within her. Despite Meishi's lack of investigative skills and reasoning, she is unable to take her eyes off him, with her face quickly turning red. From all the countless cases she came across, Rinna finally faces a mystery she cannot solve: the throbbing of her heart.