The stakes are incredibly high in the brewing courtroom stoush, with the loser likely left with millions of dollars of legal bills.
However, there is even more on the line for Harry and Meghan. On Monday this week, the Queen gave them permission to step back from frontline royal duties and for them to split their time between Canada and the UK.
The Sussexes' have found themselves at the centre of a global media storm since they announced, reportedly without giving Her Majesty, Prince Charles or Prince William advance warning, they were quitting as "senior" members of the royal family.
In the days and weeks since the "abdication" crisis erupted, Harry and Meghan have faced a growing tide of anger in both the UK and Canada over who will pick up the million-dollar bill for their security, and calls for them to pay back the $4 million of taxpayers' money used to renovate their Windsor home.
THE PRE-WEDDING FALLOUT
In May 2018, per the messages included in the Associated Newspapers documents, Thomas Markle was excited. Later that month he was set to walk daughter Meghan down the aisle in an extravaganza of a royal wedding.
On May 10 he messaged her, writing: "I look forward to trying on my shoes and see how we look, thank you for getting it ready for me, it's probably past your bedtime so have a good night I love you, Dad".
However, the next day the humiliating news broke that the Emmy Award-winning lighting engineer had staged paparazzi photos after which he is said to have spoken to his daughter and her fiance.
The documents filed this week alleged that on May 14, he then wrote to the one-time Suits star, saying "that he loved her and that he would not be attending the wedding and that he was going to make a public apology to the Claimant and Prince Harry".
Per The Telegraph, "(Thomas) received a text response from Prince Harry saying that he (Mr Markle) did not need to apologise and that he should call".
The same day the then 74-year-old went to hospital in California with heart problems and then reportedly messaged his daughter to tell her he had undergone surgery and doctors had told him not to fly, therefore would not be at her wedding.
On May 17, he received a text signed "Love M and H," "admonishing him for talking to the press, telling him to stop and accusing him of causing hurt to his daughter".
Thomas allegedly responded: "I've done nothing to hurt you Meghan or anyone else I know nothing about 20 phone calls, I'm sorry my heart attack is any inconvenience for you."
On May 19, Harry and Meghan wed in front of a global TV audience of an estimated 1.9 billion people. Prince Charles stepped in and gave the former actress away.
The documents allege that after the big day, Thomas attempted to contact the now-Duchess by text and by phone. In August, he allegedly received a lengthy, handwritten missive from her and which is referred to in the legal documents as The Letter.
THE LETTER GOES PUBLIC
The contents of the first letter was revealed on February 6, 2019, more than six months after it was allegedly sent, when five unidentified friends of the Duchess spoke to People. One "close friend" told the magazine: "After the wedding she wrote him a letter. She's like, 'Dad, I'm so heartbroken. I love you. I have one father. Please stop victimising me through the media so we can repair our relationship.'"
Days later on February 10, the Mail on Sunday published excerpts from the letter provided to them by Thomas, who said in an interview that he believed that letter "would be an olive branch. Instead, it was a dagger to the heart".
On October 1 last year, as Harry and Meghan were enjoying the end of their highly successful tour of South Africa, they dropped the bombshell news that they were suing the newspaper's parent company Associated Newspapers, alleging breach of copyright, breach of data regulation laws and a breach of her right to privacy, along with claiming that the letter was selectively edited.
'READ BY THIRD PARTIES'
The defence documents submitted to the High Court this week allege that the "letter was written and sent by the Claimant (Duchess of Sussex) with a view to it being read by third parties and/or disclosed to the public, alternatively knowing that the same was very likely".
"The Letter does not appear to contain the Claimant's deepest and most private thoughts but to be an admonishment by the Claimant of her father for failing to behave as she would have wished."
The submission also alleges that: "The Letter appears to have been immaculately copied out by the Claimant in her own elaborate handwriting from a previous draft. There are no crossings-out or amendments as there usually are with a spontaneous draft. It is to be inferred also from the care the Claimant took over the presentation of the letter that she anticipated it being disclosed to and read by third parties."
MEGHAN AND HARRY'S BIG GAMBLE
Days after the Mail on Sunday lawsuit was announced, it was revealed that Prince Harry is suing The Sun and Daily Mirror for alleged phone hacking.
Though members of the royal family have turned to the courts in the past, they have pursued legal matters sparsely given the risk they pose in terms of revealing personal details about the Windsors (the most recent high-profile legal stoush saw the Duke and Duchess of Cambridge sue over the publication of topless images of her back in 2012).
In an analysis of the current situation, the BBC's royal correspondent Jonny Dymond wrote that "the tone and content of the defence offered by the Mail on Sunday is a shot across the bows of Team Meghan.
"It is a taster of what the Mail on Sunday will try to make the court case about – not centred on copyright law and data regulations, but about Meghan's character, her credibility, and the way she treats her family."
WHAT HAPPENS NEXT?
We wait for the time being, given that no day has been set for a hearing.
Whenever this case does finally hit the courtroom, it will most likely become a global media circus and will see the Sussexes' face unprecedented scrutiny of their relationships and family dealings.
Keep in mind, this is also only the first of the couple's three court cases, meaning they could spend years – and a small fortune – making their way through the legal system.
However, this situation looks set to only escalate. Meghan's half-sister Samantha Markle, who has frequently and venomously criticised the Duchess in various interviews, told the BBC of her father, "If he is called, he will come."
