Daikichi Kawachi is a 30-year-old bachelor working a respectable job but otherwise wandering aimlessly through life. When his grandfather suddenly passes away, he returns to the family home to pay his respects. Upon arriving at the house, he meets a mysterious young girl named Rin who, to Daikichi’s astonishment, is his grandfather's illegitimate daughter! The shy and unapproachable girl is deemed an embarrassment to the family, and finds herself ostracized by her father's relatives, all of them refusing to take care of her in the wake of his death. Daikichi, angered by their coldness toward Rin, announces that he will take her in—despite the fact that he is a young, single man with no prior childcare experience. Usagi Drop is the story of Daikichi's journey through fatherhood as he raises Rin with his gentle and affectionate nature, as well as an exploration of the warmth and interdependence that are at the heart of a happy, close-knit family.

As the head of her middle school's science club, it's only fitting that Ueno is also a brilliant inventor. With devices that can convert any liquid into drinkable water, deodorize the most foul smells, or even generate dark matter to be used as a means of concealment, it seems like nothing is beyond Ueno's capabilities. However, she doesn't invent these devices for the advancement of mankind. Rather, the one force that motivates her is love, the only phenomenon she can't quite figure out. Ueno is head over heels for Tanaka, her nonchalant fellow club member. Yet, because she is too nervous to confess her love and he is too oblivious to notice her affection, her love life is completely stagnant. In Ueno's mind, if she could just expose him to perverted situations, then surely he'd get flustered and fall for her, right? Assisted by her stone-faced classmate and dedicated wingwoman Yamashita, Ueno employs her many inventions on Tanaka in a lewd manner in hopes that he may one day understand how she feels.