Anata no Koto wa Sorehodo

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 it is destiny
calling, and the two spend one night together in secret from their
spouses. Airtime himself has just begun a family of his own, with a
newborn child, but he and Mitsu find themselves continuing their affair.
While they at first enjoy what seems to be a no-strings at-tached
affair, it gradually begets subtle changes in feeling. Mitsu's husband,
Ryota, becomes aware of the affair and ups his devotion to her to keep
her. Meanwhile, Arishima's wife, Reika, is a picture perfect mother and
devoted wife, throwing this four-way relationship into an increas-ingly
complicated web of mixed emotions.