Nagasaki in 1986. Hisada, who lives with her affectionate parents and her younger brother, although there are many quarrels between couples, is a fifth grader who loves Yuki Saito and Exogini. Hisada decides to go to Boomerang Island to "see the dolphins" with Takemoto, whose house is poor and avoided by his classmates. Drowning in the sea, getting entangled in the Yankees, and having a lot of trouble, but as the friendship between the two deepens as a result of this adventure, a sad incident that presages farewell occurs.