Archibald Hamish Lethbridge-Stewart

Captain Archibald Hamish Lethbridge-Stewart was a member of the Lethbridge-Stewart family who served in the British Army in World War I. He served at Ypres in December 1914 and, following an encounter with the Twelfth Doctor and First Doctor, participated in the Christmas Armistice. (TV: Twice Upon a Time) He was the brother of Alistair Conall Hamish Lethbridge-Stewart and the great-uncle and biological grandfather of Brigadier Alistair Gordon Lethbridge-Stewart. (PROSE: Night of the Intelligence)

History
In 1902, while her husband Alistair was off on a mission, Lillian Lethbridge-Stewart, in a moment of loneliness, turned to Archie, her brother-in-law, and a night of indiscretion followed. Nine months later she gave birth to her second child, Gordon. Everyone believed the father to be Alistair. (PROSE: The Note)

He served at Ypres in December 1914. On Christmas Day, near Saint-Yvon, Archibald and his men were laying field telephone cables at the front line when they were caught up in a German bombardment and a follow-up attack. (PROSE: Twice Upon a Time) Archibald was stuck in a crater with a wounded German soldier, where both men held each other at gunpoint. Archibald stated that he wouldn't kill the German soldier except in self-defence, but he didn't speak German, nor the German any English. Before they could kill each other, Archibald was abducted by the Testimony in order for them to record his memories. Due to an error in the timeline caused by the First Doctor and the Twelfth Doctor resisting regeneration at the same time, Archibald was returned to Antarctica in December 1986 instead of Ypres, where he became the subject of a conflict between the two Doctors and the Testimony, who wished to return Archibald to his own time.

After the truth about the Testimony was revealed, it was agreed that Archibald would be returned to his own time period, with the Doctors acting as his escort. On the return trip aboard the Twelfth Doctor's TARDIS, Archibald expressed regret that he would never see his wife and sons again to the Doctor. After returning to Ypres, Archibald was informed that his memory of the entire experience would be erased while a perception filter would keep him from seeing the two Doctors. The First Doctor expressed regret that "the universe generally fails to be a fairy tale", and Archibald revealed his name and requested that the Doctors check up on his family. After both Doctors promised to fulfil his request, Archibald returned to his spot in the crater, causing time to resume and his memories to instantly be erased.

As Archibald and his German opponent prepared to shoot each other, both men heard Christmas carolling on both sides, as a "Christmas truce" began. Laughing in relief, Archibald and the German soldier ended their standoff and Archibald called for medical attention for the man. Privately the Twelfth Doctor explained that he adjusted Archibald's timeline by a couple of hours to save Archibald and his opponent's lives. Some hours later, as the truce began to end, Archibald noticed the Twelfth Doctor standing on the field and saluting him, causing Archibald to confusedly salute back. (TV: Twice Upon a Time)

Archibald visited his brother, Alistair Conall Hamish Lethbridge-Stewart for Christmas in Carmunnock, 1917. Archie's train was delayed, so he had to take an alternate train in London, arriving on 26 December instead. He had a drink with his brother to celebrate both their offspring's future. (PROSE: What's Past is Prologue)

Original timeline
In an alternate timeline, Archibald and his German opponent killed each other on the battlefield. (TV: Twice Upon a Time)

Behind the scenes

 * According to Mark Gatiss, who played the character, Archibald is the grandfather of Brigadier Alistair Gordon Lethbridge-Stewart., an identification tacitly supported by Twice Upon a Time's writer Steven Moffat. However, Archibald previously appeared in the novel Night of the Intelligence, part of Candy Jar Books' Lethbridge-Stewart novel range, where he was identified as the Brigadier's grandfather's brother.
 * On 26 December 2017, the Haisman Estate put out a statement saying the character was the Brigadier's great uncle. Three days later, these differing behind-the-scenes interpretations were finally resolved with the short story What's Past is Prologue. Released on 29 December, this story implied that Archibald was, in fact, secretly Gordon Lethbridge-Stewart's father. As Andy Frankham-Allen said on Twitter later that day, "After the last few days, I suggested a new compromise to the Haisman Estate and they agreed to it. So, currently, it's just an implication. Now fans can accept whichever, and both could well be right." Archibald's status as Gordon's biological father was confirmed by the short story The Note.
 * In The Invasion from Mars, an adventure module commissioned by FASA from J. Andrew Keith for ''The Doctor Who Role Playing Game but ultimately unproduced, the Brigadier's grandfather MAJ Archibald Lethbridge of the Royal Horse Artillery would have played a key role in repelling the Martian invasion of Earth.