Onodera Nanami, elated from her third date with her new boyfriend, arrives at her apartment to find an strange, but extremely handsome boy with big ears welcoming her home as "Mistress!" Bewildered, Nanami soon finds out her genius older sister decided to test her new companion robot prototype called "D-46" on her…who happens to be this cute, puppy-like boy at her doorstep. Will Nanami stay true to her boyfriend, or will this android steal her heart away?