On Valentine's Day, Sasami Tsukuyomi, an unmotivated shut-in, attempts to give Valentine's chocolate to her doting brother, Kamiomi. However, having dedicated so many days to Sasami, Kamiomi is unaware of Valentine's and ends up giving his chocolate away, frustrating Sasami as she views him via a surveillance system. Sasami makes various attempts to go after him, but is thwarted by the summer heat. When he eventually learns of Valentine's Day from the three Yagami sisters, Tsurugi, Kagami and Tama, he becomes determined to buy Sasami some chocolate. Things soon take a turn for the weird when the entire world starts turning into chocolate as the result of some powerful gods that the Yagami sisters, who possess unique abilities and skills, fight against. After the gods are defeated, Sasami is left frustrated after having been encased in chocolate.