Yamada is a castaway on an island where no ships pass by, and he cannot remember why he is there. After discovering his sports bag, he assumes he was on a school trip. With only seven days' worth of food, he eventually decides to build a boat to leave the island. He believed he was alone until he found a girl named Imaise Chika eating his food. They then decide to continue carving the boat together. Since the boat has room for only one person, Yamada decides to set off for help, but returns upon noticing that Imaise has put her share of rations in his bag and that they are mostly untouched. After returning, he notices she made the oar out of a fishing lance, and he decides they can live on the island longer.