Amago Tsunehisa transfers into a daycare and makes friends with a boy named Shimizu, who decides to help fulfil her dream of building a large sandcastle in the sandbox. Tsunehisa starts using her wisdom to slowly gain control of the various territories of the sandbox. However, another classmate named Misawa tells on the teacher, who forbids Tsunehisa from using the sandbox until after the weekend. The next week, Tsunehisa attempts negotiations with Misawa over how to split turns on the sandbox, but ultimately fails. Misawa decides to challenge Tsunehisa to a game in which they take turns to remove sand from a pile with a stick in it without causing the stick to fall. Although Misawa initially has the advantage, Tsunehisa manages to take into account the scheduled nap time, during which the wind knocks over the stick, securing Tsunehisa's victory. With the sandbox hers, Tsunehisa and the class work together to build the sandcastle of her dreams. However, moments after completion, Nobunaga ends up wrecking it when she comes to claim Tsunehisa's secret treasure, prompting swift vengeance from the class.