Rentarou gets a ZING!! from a girl in the chemistry lab on the way to a pancake buffet with his girlfriends, getting dragged away before he can identify girlfriend #5.
The next day after school, Rentarou returns to the lab, finding an 8-year-old girl working on an experiment. The girl introduces herself as Kusuri Yakusen before providing Rentarou a cup of tea. She then prepares a human magnetization drug, which she promptly goads Rentarou to drink.
After forcing him to drink the drug and explaining the side effects after the fact, she drinks a dose of the drug with Reverse Polarity, causing the two to get stuck hugging each other. (If the doses were the same polarity, Kusuri would be upside-down from Rentarou.) Fortunately, the effect only lasts for ten minutes. Unfortunately, Kusuri has to pee. Since she routinely wears diapers to avoid interrupting her experiments, she simply relieves herself right then and there.
After the drug wears off, Kusuri starts going through all the drugs she invented as Rentarou tries to get his mind back on track. This is when Kusuri reveals that she spiked the tea she served him with a Love Potion due to having fallen in Love at First Sight with him. Rentarou immediately tries to regurgitate the drug, not wanting to accept a confession under the influence of a drug, but his body soon becomes overwhelmed. Kusuri proceeds to administer a neutralizer, having to do so via mouth-to-mouth due to the body spasms.
After Rentarou recovers, the two of them realize that he still thinks Kusuri is cute even after being given the neutralizer, leading her to realize that the Love Potion overwhelmed him due to him already having strong feelings for her. She then confesses while prostrating herself.
Upon standing back up, Kusuri's age has more than doubled, giving her the same appearance as the girl Rentarou saw the day before. She explains that she's actually 18 years old, but a failed attempt at an immortality drug left her with an 8-year-old body that can only be reverted for ten minutes at a time with the neutralizer. (She swallowed some of it while administering it to Rentarou.) Realizing she really is his soulmate, Rentarou returns Kusuri's confession while prostrating himself.
Kusuri is ecstatic about Rentarou accepting her until she realizes she needs to change diapers.
Tropes:
- Brain Bleach: Rentarou asks Kusuri not to talk about diapers in her older form.
- Does This Remind You of Anything?: Kusuri mentions that if she and Rentarou drank the same polarity dose of the magnetization drug, they would be stuck upside down from one another and would get chewed out for it, since the manga isn't 18+.
- Imagine Spot: Rentarou has one of Nano copying Kusuri's Verbal Tic.
- Intimate Healing: Kusuri has to administer the neutralizer to Rentarou via mouth to mouth due to his body spasms.
- Love Potion: Kusuri gives Rentarou one in a cup of tea. He proceeds to vomit it out when he learns about it.
