A fourteen-year-old middle school girl named Nico Hayashi possesses a remarkable and self-taught talent: she can mimic any voice she hears and identify subtle emotional nuances that others miss. She initially uses this ability to intervene in dangerous situations, such as manipulating men who solicit young girls for compensated dating, which gives her a deep, somewhat cynical understanding of adult hypocrisy. Her life changes when an aging gangster recognizes her unique skill and hires her to work as an unofficial spy and troubleshooter for various odd cases.

While investigating her first assignment, Nico encounters Iichiro Sudo, a twenty-something man-child who works at a telemarketing club. Sudo is obsessed with collecting robot toys and watching the anime RoboMax, a passion he indulges with an almost religious fervor. Finding his geeky enthusiasm ridiculous yet occasionally useful, Nico dubs him Robo, and he becomes her reluctant partner. Where Nico is sharp, precocious, and emotionally guarded, Robo is naive, earnest, and prone to crying or shouting catchphrases like Max Punch at inappropriate moments. Despite his childish demeanor, he has a car and his own apartment, making him an oddly practical asset.

Together, Sexy Voice and Robo take on a series of short, loosely connected, character-driven adventures. The cases are often melancholy and bittersweet, revolving around lonely or desperate people. One early episode involves a man who can only remember the past three days of his life, carrying a bag of trash as proof of his existence. Another features a ghost that seems to be haunting Nico's mother, only to be linked to a joyride with Robo. While solving these mysteries, Nico often uses her voice to confront suspects or give clients closure, while Robo defuses tense situations by relating them to his robot collection, sometimes talking people out of suicide through absurd anime analogies.

The series is set in contemporary urban Japan, moving between Nico’s school, her family home, Robo’s cluttered apartment filled with plastic models, and the various mundane locations where their cases unfold. A significant part of the narrative arc involves the growing, if unlikely, bond between the two leads. Although Nico frequently berates Robo for his otaku obsessions and his desperate, unsuccessful attempts to find a girlfriend, she continues to involve him in her work, perhaps because he is one of the few people who does not judge her or try to exploit her talent. Underneath the comedy lies a darker meditation on life, loneliness, and human connection, forcing Nico to confront realities far beyond her years.

As the eleventh and final episode approaches, their employer decides to retire and travel, effectively dissolving their spy agency. On his last day, Nico and Robo walk together through a park. Nico receives a phone call and tells Robo she will see him tomorrow. A final voice-over, however, reveals a poignant and abrupt ending: they did not meet again. The last time Nico saw Robo, he was leaning against a wall staring at the stars. She says she could not find her voice to call out to him, and their paths never crossed again, ending their strange partnership on a note of quiet, unresolved loss.