My girlfriend is a blue-collar worker/What she did, what was done to me/A plan to capture the voluptuous wife/My wife was stolen by that bastard

The year of 2012, late autumn. Tanaka Masashi, an architect, and Tanaka Misaki, a beautician, are in their third year of marriage. Masashi is invited with his wife on a friendship trip to an onsen by Aida, an executive director of the construction company, a frequent business partner of Masashi's firm.
Laying in the open-air bath with Misaki, Masashi starts recalling the beginning of their love.

The year of 2006. Masashi spends his time working in a convenience store three days a week. He is a 19 year old part-time worker. Being at a loss because of a sudden breakdown of the only source of amusement in his room, a television, he is recruited against his will by Aida to work on the construction site. And there he encounters Kaitou Misaki, the 19 year old chief of his team.

The year of 2016. During the time when Misaki starts systematically being late in her way from work, Masashi finds an unexpected item in his own bedroom. Moreover, Misaki suddenly shuts down her beauty salon, although it was doing well, and gets a job at Aida's construction company.
In late autumn, Tanaka wakes up in the middle of the night at a hot spring inn where he went on a friendship trip, just as he did four years ago, and witnesses an unbelievable scene.

[Translated from Getchu]