Mitsu is a 29-year-old woman who married “the number 2 man in her life,” but has been unable to forget her first love, Arishima. When she accidentally encounters him after several years, she feels that it is destiny, and the two spend one night together in secret without their spouses knowing. Arishima himself has just begun a family of his own, with a newborn child, but he and Mitsu find themselves continuing their affair. While at first they enjoy what seems to be a no-strings-attached affair, it gradually begets subtle changes in feeling. Mitsu’s husband, Ryota, becomes aware of the affair and ups his devotion to her in an attempt to keep her by his side. Arishima’s wife, Reika, a perfect mother and devoted wife, makes this four-way relationship an increasingly complicated web of mixed emotions..