When Shiori Ito arranged to meet Noriyuki Yamaguchi for dinner at a Tokyo izakaya (bar) in April 2015, she was hoping for advice on her fledgling career as a journalist and, perhaps, a recommendation for a job. Yamaguchi, the former Washington bureau chief of Tokyo Broadcasting System, a respected TV network, was well connected. He had written a favourable biography of the prime minister at the time, Shinzo Abe, whom he counted as a friend.

Instead, the meal would mark the start of Ito’s private hell. She tells of how, despite indicating she wished to go home, the then 26-year-old was piled into a taxi and taken to a hotel, where Yamaguchi, more than two decades her senior, raped her. Two years later, against the advice of her family and the authorities, Ito went public with her allegations, followed by the publication of her book about the case and the shroud of secrecy hanging over sexual assault in Japan.

In the nine years since, Ito’s story has become a study in courage and a catalyst for changes in the way Japanese officialdom views sexual assault. Her testimony, though, has come at a price. For many women, she was the face of the #MeToo movement in Japan. But Ito also became a lightning rod for misogyny and victim-blaming, her critics casting her as an overly naive young woman who had let down her guard.

This month, Ito will have what she hopes will be her final say with the release of the documentary Black Box Diaries, her directorial debut. It is an intensely personal retelling of the rape and its aftermath. Drawing on a vast number of diary entries and recordings – some of them secret – the documentary, which premiered at this year’s Sundance film festival, is a study in hope and crushing disappointment; of legal victories and public vilification; of determination and moments of such intense personal distress that viewers, once they have addressed their simmering rage at her plight, can only speculate about the nature of the material Ito decided to leave out.

As she prepared to leave for the UK, where the film will be released on 25 October, Ito had still not found a Japanese distributor. “We are having difficulties,” she says. “It’s not a big surprise, but it’s unfortunate. But we’re trying our best to find a way.” The reasons behind distributors’ reluctance are opaque, but it has not been lost on Ito that, the controversial subject matter aside, Yamaguchi remains a prominent figure in journalism and has never faced criminal charges for the allegations, which he has consistently denied. “There are no legal issues [with the documentary’s content], but it seems like potential distributors and theatres are scared of the risks of showing it,” Ito says. “It’s still politically sensitive.”

The documentary opens with a trigger warning for viewers, advising them to take a moment. What follows is a troubling account of a stream of missed opportunities on that night. CCTV footage from outside the hotel shows Ito, then an intern at Thomson Reuters in Tokyo, being dragged out of the taxi she shared with Yamaguchi to the hotel. We hear the taxi driver – who later testified on her behalf in a civil case – ignore her requests to be driven home. We see Yamaguchi and Ito, clearly intoxicated and unsteady on her feet, pass hotel doormen into the lobby.

But their failure to intervene pales beside the treatment Ito says she was subjected to by the police, who initially discouraged her from filing a report, warning her that it would ruin her career. She was interviewed by male officers – there were no women on duty – including one who asked her to re-enact the rape with a lifesized doll while he took photographs.

Ito eventually filed a criminal complaint, which police handed to prosecutors. She was led to believe Yamaguchi’s arrest was imminent. And then … nothing. A court-issued warrant for his arrest was dropped, reportedly on the orders of Itaru Nakamura, then the acting head of the Tokyo metropolitan police department and a close confidant of Abe and Yamaguchi; in a 2017 magazine interview, Nakamura admitted his role in the case. Yamaguchi continued to deny the rape allegations, claiming in an open letter to Ito published in a conservative magazine that he had not, as she had asserted, drugged her or forcibly dragged her into the hotel room.

Much has happened since Ito’s memoir, Black Box, was published in Japanese in 2017 (an English edition appeared in 2021). In 2019, she secured justice, of sorts, when a court ordered Yamaguchi to pay her ¥3.3m (£23,000) in damages in a civil suit and dismissed his ¥130m countersuit.

“When I wrote the book, I didn’t feel OK including my emotions as a survivor and thought it would be easier to protect my mental health if I just pretended that I was covering a story as a journalist,” she says. “There was some distance between me and the story. But for the documentary I had to go through old diaries and recordings … there were things I had forgotten because they were too traumatic, so it was like reliving everything. Editing it was like a form of exposure therapy.”

How does she feel when she sees Yamaguchi’s name in the media? “Before, when I saw him on TV, I felt terrible … It makes me scared … I’m not a judge, but in our civil court case, the ruling stated there was no consent – so it was rape – but it’s acceptable for him to walk around as if nothing happened.”

Ito has successfully sued one of her tormentors. In 2022, Mio Sugita, a conservative lower house member from the ruling Liberal Democratic party, was ordered to pay Ito ¥550,000 for “liking” 25 tweets that slandered Ito, including one that accused her of attempting to sleep her way into a job.

Her case has also resulted in legal changes in Japan, notably last year’s revision to a rape law stretching back more than a century that had forced victims to prove that they had been incapacitated by violence at the time of the assault. The definition of rape was broadened from “forcible sexual intercourse” to “non-consensual sexual intercourse”.

Ito has begun dismantling the taboo around sexual violence, but almost a decade on she says Japan – where, according to a 2017 survey, only 4% of women report sexual assault – still has a long way to go. “There is a #MeToo movement in Japan, although it doesn’t feel like it was as strong as in South Korea and maybe China. There is so much more that Japan has to do … just look at the whole Sugita thing … this was a woman in a position of political power who was basically calling me a liar.”

Ito, who has spent long periods in the UK since she went public, in part to slow the avalanche of hate that followed, hopes that Black Box Diaries will mark the end of her story and the beginning of a more honest discussion about sexual violence. “I still have ups and downs, but they’re getting shallower,” she says. “I’m 35 now and have spent most of my youth dealing with this. I don’t know any different, though – what it would have been like to have had a ‘normal’ life in my late 20s.

“I’d like people who watch the documentary to feel that we all have the power to tell a story. And I hope it empowers them to believe in themselves and reach out.”

• Black Box Diaries is released on 25 October