Shiho Nishina (Aimi Satsukawa), who has a son, and her husband Hirosuke Nishina (Kenei Chiga) are bothered by their lack of interest in the nightlife of the couple. One day, Hirosuke suddenly proposed to Shiho that they go to a couple exchange party together. Shiho disagreed at first, but was finally convinced by her husband's insistence. At first, Shiho reluctantly participated in the couple exchange event called the Midnight Aquarium with the mentality of just going to see it, but she gradually discovered the beauty of exchanging couples in the process. Where will this story of self-discovery with a dangerous atmosphere go?