It seems like ages ago that Sakuta Azusagawa and Mai Sakurajima found each other. But their unique relationship is put to the test when Sakuta's first great love Shoko Makinohara shows up and unceremoniously moves in with him! But that's not all, because there seem to be two different versions of her: in addition to the adult Shoko, Sakuta meets a younger version who is still in middle school and apparently suffering from a serious illness. Sakuta would like to help her, but to do so he first has to find out what the adult Shoko is trying to hide from him...