K is a robotic crime investigator assigned to the Special Scientific Investigation Division of the Tokyo Metropolitan Police. Created by the brilliant scientist Saori Kirishima to oppose the criminal organization BAD, K possesses a human-like personality and emotions, expressed through his changing eye color: yellow at rest, red in anger or combat, blue when sad or low on power. When off duty, he wears a yellow hunting cap and red blazer to blend in with humans, but in battle he sheds his outer clothing and enters a combat mode with five times his normal strength. His primary weapon is a chest-mounted Robot Destruction Gun.

K is partnered with veteran detective Daizo Shiba, a gruff, intuitive officer with twenty-five years of experience who initially despises the robot because he lost his wife in a traffic accident and resents machines replacing human intuition. Shiba scornfully calls K a "tin-can bastard" and a "mechanical doll." The younger detective Tsuyoshi Shinjo, a hot-blooded and physically capable officer, acts as a mediator, treating K without prejudice and later developing a romantic relationship with Shiba's older daughter, Nami.

The series begins with a locked-room murder case that appears impossible for a human to commit. K appears before Shiba and Shinjo and declares that the culprit must also be a robot. This launches a 26-episode campaign against BAD, a clandestine organization led by a disembodied voice (also named BAD) that deploys a series of specialized robot criminals, each named after its gimmick: Wakkaman, Tenagaman, Kaminariman, Dennetsuman, Missileman, Gatlingman, and many others. These robots commit assassinations, sabotage, and terror attacks across Tokyo.

As the investigation deepens, K’s origin is revealed: he was built by Saori Kirishima, whose brother George Kirishima (also known as BAD's human agent) has gone mad and created the robot army to spread chaos. The giant flying fortress called Mother serves as K's recharging station and command center. Over the course of the series, Shiba gradually warms to K after being saved multiple times by the robot's selfless actions, culminating in Shiba telling K, "Let's work together forever."

A major turning point occurs in episode 22, when K's body is upgraded with additional cannons, reflective head mirrors, and shoulder and abdominal missiles. By shouting "Blow Up," his entire body turns red and he enters a full battle mode with enhanced jumping power and deep-sea/space capability. This upgrade is necessary to match BAD's increasingly powerful robots.

The final arc reveals BAD's true nature and leads to a confrontation on Mars, where BAD is ultimately destroyed. Throughout the series, K not only fights crime but also composes poetry and tries to understand human emotions, proving that a machine can act with compassion, courage, and justice. The stories blend police procedural elements with science fiction, exploring themes of humanity, prejudice, and coexistence between man and machine.