When she finally broke off the relationship, she made an official complaint to the university and sued the professor for sexual harassment. Her argument: that he had exploited his position as her supervisor when she was 23 to groom her for sex, assault her and then fundamentally hold her under his sway for years.

But in a twist, she also found herself sued by the professor’s wife, accused of adultery and causing mental distress under Japan’s civil code, which views extramarital relationships as an infringement of the marriage contract.

In the end, the wife won nearly $20,000. The professor was fired last year for, the university said, conducting an “inappropriate relationship.” But the young woman lost her case when the court ruled that the professor had never forced her to do anything against her will.

The story of Meiko Sano, now 38; her professor, Michio Hayashi, 63; and his wife, Machiko, 74, highlights the tangled state of sexual power dynamics in Japan, where women rarely bring — much less win — cases for sexual harassment, and where the #MeToo movement has yet to take hold as it has in the West.

Ms. Sano knew her sexual harassment suit against Mr. Hayashi was a long shot. But she went through with it, she said in several interviews, to show how she had experienced “psychological abuse like grooming and gaslighting that Japanese are really not sure about.”