Asuha has always had a crush on Mizuki, but hasn't known how to express it. She wakes up one day shrunk down to the size of one of his erotic figurines, naked and in his bedroom. Sexual events ensue while she and Mizuki try to figure out how to turn her back to normal. There's an emphasis on how it would be strange for him to like her since he has a preference for small-breasted girls/dolls. I didn't really care for the last three chapters.
A recurring technique this manga uses is having Asuha get some sort of stimulus in the real world while passed out and then fantasizing about sex acts with Mizuki which would involve similar stimulus. I think every chapter has her slipping into a fantasy at some point.