Midori Oonuki and her husband have been married for 40 years, maintaining a deeply affectionate relationship as a devoted couple. With their son now independent and the husband retired, they've been living a quiet, peaceful life together. However, an unexpected gift from their son—a hot spring trip to celebrate their 40th anniversary, the Ruby Wedding—reignites the spark between them. They've never taken a trip together as a couple before. Ever since their son was born, they gradually lost intimacy, falling into a sexless marriage and becoming little more than life partners. But in the special setting of this journey, they reconnect emotionally and rediscover a new bond as husband and wife.