Iroha, a masculine girl, and her brother Hifumi, a feminine boy, are transferring to a different middle school and move in with their grandfather. After their first night, however, they discover that their grandmother's spirit swapped their genders, figuring that male!Iroha and female!Hifumi have better chances of giving her grandkids. The supposed cure is to act as closely to their original gender's stereotypes as possible, so either way, the grandmother wins.
!!This manga provides examples of:
* CrotchGrabSexCheck - How Ryou determines Iroha is male, leaving Iroha on the ground foaming at the mouth.
* MasculineGirlFeminineBoy - Well... for the first few pages, at least.
* MeaningfulName: Iroha (analogous to "ABC") and Hifumi (123).
* SureLetsGoWithThat: When Iroha went to buy a bra and was caught by people who only met her after she was turned into a boy, they thought it was because she lost some bet. She gladly allowed them to think they were right.