In the summer of 1989, four kids in Tokyo witness the emergence of the turtle kaiju Gamera, who bravely stands up against giant human-eating monsters.

It's summer of 1989 for sixth graders Boco, Joe, and Junichi, their last summer vacation as elementary school students, and a vague anxiety about the future in each heart. Brody, son of the commander of the U.S. Forces in Japan, appears before the three of them, stealing their one-year-accumulation of cash. Angry, Boco leads the strategic plan to get their money back, but when the strategy is about to be carried out, a crisis comes to the town. The monstrous Gyaos suddenly attack Tokyo. Four children stand in a town that has been ruined by Gyaos. When the Gyaos zero in on them, a huge, monster turtle appears. Its name is Gamera. This was the beginning of "Monster Summer" for the four of them. As monsters appears one after another, Gamera fights, even though injured, and the boys witness the legend.