This is a dispute that says much about the age we live in, involving as it does yobs on a train, Twitter, two figures from very different versions of left-wing politics and, ultimately, libel lawyers.
First, the protagonists.
In one corner is Paul Embery (Twitter followers: 119,000), a firefighter, trade unionist, author of Despised: Why the Modern Left Loathes the Working Class, and supporter of what you might call traditional working class values.
He believes that immigration needs controlling, campaigned for Brexit and scorns the idea that men can become women simply by saying that's how they feel.
In the other corner there's Jo Grady (Twitter followers: 56,000), General Secretary of the University and College Union and very much of the modern Left.
She voted against Brexit and has boasted that her Twitter account has software to block terfs - terf being an insult directed at people opposed to the idea that you can change your biological sex.
These two have clashed over an incident last August when Mr Embery got a train from London to Norwich with his children aged 12 and 15.
"A group of women sitting directly behind us were consuming alcohol and behaving in a loud and obnoxious manner," he says.
"One woman in particular was repeatedly using sexually-explicit profanities."
Many of us would have stared out the window, pretending nothing was wrong, but Mr Embery politely asked the woman who was swearing to tone down her language because children could hear.
That request was met with a torrent of abuse.
So he found the guard, who succeeded in getting the women to behave - until he left the carriage and then the swearing and abuse started again.
So Mr Embery resorted to that modern weapon, the mobile phone, and began openly filming them, hoping this would moderate their behaviour.
It didn't. He says he was called sexually explicit insults, threatened with being knocked out and getting a beer bottle in the face.
He then tweeted a photograph of the women, hoping that the train operator might take action, and the tweet went viral.
At which point you might be thinking, what a hero. Here was someone who did what we'd all like to do if only we had the courage.
Not everyone saw it like that, including Jo Grady.
“Grow up Paul and take a day off bullying women and pretending to be outraged for clicks. It’s pathetic at any age, but especially yours," she tweeted.
“It’s creepy to record young women on the train, share that video, and lie about them on social media for clout,” ran another of her tweets.
For Mr Embery, this went way beyond a passing Twitter spat. He was being publicly called a creep who bullies women, and also a liar.
That last claim was particularly odd because Dr Grady was not on the train and in no position to question the accuracy of his account of the incident.
"My motive in tweeting about the incident was to report anti-social behaviour," says Mr Embery.
"My tweet highlighted a very real problem that ordinary, decent passengers face every day on public transport."
He says that he wrote to Dr Grady asking her to apologise and withdraw her allegations.
She refused.
He offered to show her the footage he shot which would support his account, but she did not want to see it.
So, enter the lawyers.
Mr Embery went onto crowdjustice.com to find the £24,000 needed to sue Dr Grady. It took him just eight days to raise the money.
"The only reasons for bringing this claim are to stand up for what is right and to obtain vindication for the harm my character has suffered," he wrote on the site.
"Any damages I receive will be donated in their entirety to groups campaigning to defend women's sex-based rights and the integrity of women's sports.
"I would not keep a penny of damages for myself."
Dr Grady is no stranger to controversy. She came to national attention when she became involved in the shocking affair of Kathleen Stock at Sussex University.
Professor Stock sides with the idea that a person's sex is biological fact that cannot be changed and wants women's spaces such as changing rooms and sports teams protected.
For this she earned the fury of a hard core of student activists who accused her of transphobia and demanded that she be sacked.
The campaign against her became so threatening that she was advised to install CCTV at home and have security guards when on campus.
Professor Stock was a member of the University and College Union but could not get the support of her union just when she needed it most.
Instead the union demanded an urgent investigation into "institutional transphobia", and Professor Stock was duly hounded out of her job.
Dr Grady has not responded to my invitations to comment.
Ignoring a libel writ might not be so easy.
Investigate@mirror.co.uk
