The Bliss of Mrs. Blossom (1968)

Harriet Blossom (Shirley MacLaine) is married to Robert Blossom (Richard Attenborough), a businessman who'd rather spend the night at his bra factory than at home with her. One day, Harriet's sewing machine breaks, so Robert sends a repairman, Ambrose (James Booth), to fix it. It's lust at first sight for Harriet, who convinces Ambrose to hide out in the attic for a tryst. When her new beau shows no desire to leave, the pair begin a years-long love affair right under Robert's nose.