K, a robot detective with an expressionless mask and a blue-black steel body, belongs to the newly established Special Investigation Unit of the Metropolitan Police Department. He is a fighting machine equipped with powerful weapons and advanced scientific investigation mechanisms, yet he possesses a capacity for thought that rivals that of humans and, beneath his cold exterior, a gentleness that surpasses them. K is partnered with Shiba, a veteran detective who relies on instinct and legwork. Shiba has harbored a deep distrust of machinery since his wife died in a traffic accident caused by a mechanical failure, and he resents being paired with an android, frequently reminding K that he is just a machine.

The story begins with a locked-room murder case that appears impossible for a human to have committed. K identifies the culprit as another robot, setting the pattern for a series of seemingly unrelated homicides. As Shiba and K investigate, they discover that every robot involved in the crimes is linked to a shadowy organization called R.R.K.K., short for Robot Rental Kabushiki Kaisha, which leases specialized robots to criminals for profit. Shiba grows suspicious that K himself may have been created by R.R.K.K. and planted in the police force as a spy. To clear his name, K leads Shiba to his creator, a mysterious entity called Mother. Mother rejects K, deepening his sense of isolation.

As the investigation progresses, the duo uncovers the story of Dr. Kirishima, a robotics authority who refused to develop weapons for the military during the war and was tortured into near death before vanishing with his wife. Kirishima is revealed to have had two children, whose fates become intertwined with the conspiracy behind R.R.K.K. Meanwhile, K encounters a blind woman named Yuki Kaori, who cannot see his mechanical appearance and accepts him for who he is. K develops a tender, unprogrammed affection for her, writing poetry in his spare time and wrestling with emotions that were never part of his design. Throughout the narrative, K operates within the bounds of Asimov's Three Laws of Robotics, but he increasingly demonstrates behavior beyond his original programming, forming bonds and making choices that hint at an emerging sense of self. The story culminates in K's confrontation with the true leader of R.R.K.K. and his struggle to reconcile his machine nature with his longing to connect with humans.