Chinami moves to a new apartment with her father. Her parents are divorced and her father has found a new lover who also lives in the same apartment next door. The lover also has a daughter that's the same age as her. Chinami wants to be friends but is a bit awkward to do so. Later she starts developing feelings for her other neighbor who's in the same class as her and who's also a childhood friend of her possible future sister.
Source: MAL