Single mother Ayukawa Rui raised Masaharu on her own. Masaharu's father already had a legal wife, so Rui struggled alone to raise her son, eventually seeing him through to graduate school. After finishing her role as a mother, she received a call from Masaharu's father after more than a decade. His wife had passed away years earlier, he had no heir, and now he wanted to formally marry Rui as his new wife. A few days after hearing this proposal, Masaharu invited her on a hot spring trip. Burdened with uncertainty about the future, Rui got into Masaharu's car and took a new step forward.