A young genius creates a sentient sex bot, but in his overzealous efforts creates what appears to be an artificial person, with emotions and desires... and proceeds to fall in love with her. To assuage his doubts, a second prototype, Unit #2, is created, given temporary free will, and dropped off into the world without memories or instructions, for autonomous testing.

Furthermore, though aware and open about the fact that she is a robot, she believes she is a general-purpose family-oriented household product, intentionally left unaware of her sex bot origins and some of her functionality. Her purpose: to see if she builds real human relationships with genuine emotional bonds.

Left laying in the street, she is "rescued" (and grudgingly taken in) by a college student that has become disillusioned and distrustful of women. She vows to forever remain by "Master's" side... this is a tender love story of the powerful and unbreakable bond that develops between young bachelor Masaru ("Master") and Unit #2 *("Yuu").