Tremendous progress has been made in technical advances. Various networks have taken strongholds with each new generation. Though not nearly advanced as humans, two-legged robots are utilized in many practical areas throughout the world. Amid worldwide peace, the human race remains unchanged. Kazuki Tomonaga is a robot made by an entity called "HIKARI" for the purpose of "GLOBAL ILLUMINATION". He's created with ingenuity as precise as that of a human being. Kazuki is commanded to research the concepts of "human emotion". Along the way he meets various girls and has exchanges with them. In their "exchanges" his "curiosity" gradually turns into "affection" and his mission becomes more intense. He gradually starts to build his own philosophy, not knowing he will be forced to make crucially important decisions in the near future.