18-year-old Yuu became pregnant and gave birth to a son named Tatsumei. The father disappeared as soon as he learned about the pregnancy, and against everyone's objections, Yuu raised Tatsumei on her own. After successfully raising him, Tatsumei entered university, and as life settled down, their relationship grew less like mother and son and more like close friends. They enjoyed shopping and traveling together, sharing endless conversations every day. One day, during summer break, Tatsumei planned a hot spring trip and invited Yuu.