Mari, Hagino, Michiko, and Akane go to Hiroko's house one day during the summer vacation, and enjoy swimming in the ocean. However, when Hagino goes to retrieve a shell, it begins raining, and Mari desperately calls out for Hagino. Meanwhile, Hiroko's pregnant sister goes into labor, and the four girls are forced to take care of Hiroko's baby niece in the process. Although they encounter problems, such as changing the baby's diaper and feeding her, they manage to calm her. Later, Hagino gives Tsubael the shell she found as a gift. As Hiroko returns to the house to retrieve the baby, she finds the four girls asleep, Mari and Hagino next to one another.