The name of the movie (The real estate agent secretly fucks the customer's wife) is probably a bit opposite because it is the wife who betrayed her husband to secretly "go to bed" with the real estate agent.
During a trip with her husband to buy an apartment, Matsui fell in love with the real estate agent, although he was old, she still had something to love. Matsui made love with his eyes, but perhaps the other man didn't understand that he should leave elsewhere, or he understood but because her husband was there, he pretended to ignore it; it's not over yet...
The next day, when the two parties had reached an agreement, the wife took the initiative to tell her husband to go to work, she didn't need to take time off from the company, she would go to the other male employee's office alone to sign the contract, of course Matsui didn't want her husband to come with her, they all had their own reasons. This time the wife does not want anyone to disturb her "private affairs" with Mr. bds, and the male protagonist in that "disturbing" role is none other than her husband - who she was bored with in the pillow.