The story is about the adventures of a 14-year-old girl named Nico Hayashi who uses her talents of changing and altering her voice to manipulate men over the phone who want to participate in Enjo kōsai. Through this she learns a lot about human nature, and gains a keen understanding of peoples through their voices. An aging gangster notices her talents and decides to hire her to solve various cases. While Nico is on her first case, she meets with a geeky man named Iichiro Sudo, who has an obsession with robot toy models. This obsession that leads Nico to call him "Robo," and the two become an unlikely team. After completing her first case Nico proclaims herself "Sexy Voice" and a variety of short loosely linked character-driven adventures ensue.