Archibald Hamish Lethbridge-Stewart

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 Lethbridge-Stewart and the great-uncle of Brigadier Alistair Gordon Lethbridge-Stewart. (PROSE: Night of the Intelligence)

History
In 1902, while her husband 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 where on Christmas, Archibald was stuck in a crater with a wounded German soldier. As the two men held each other at gunpoint, Archibald stated that he wouldn't kill the German soldier except in self-defence but recognised that they couldn't understand each other.

In the moment before they killed each other, Archibald was abducted by the Testimony in order to record his memories. Due to an error in the timeline caused by the First Doctor and the Twelfth Doctor resisting regeneration at once, Archibald was returned to Antarctica in December 1986 instead of Ypres. Archibald became the subject of a conflict between the two Doctors and the Testimony who wished to return Archibald to his own time. At one point, Archibald stated that while he was unafraid of death before, now that he knew he would die, he was scared again.

After the truth about the Testimony was revealed, the Twelfth Doctor agreed that Archibald must be returned to his own time period. However, the Twelfth Doctor requested that he and his predecessor do it as it was their fault that Archibald had been displaced in time. On the return trip aboard the Twelfth Doctor's TARDIS, Archibald expressed regret that he would never see his wife and sons again. 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 fulfill 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 caroling as the Christmas truce began. Laughing, Archibald and the German soldier ended their standoff and Archibald called for medical attention for the man. Watching unnoticed, the Twelfth Doctor explained that he adjusted the time period Archibald was returned to so that it was a couple of hours later when the truce began, saving both men's lives. The Twelfth Doctor's meddling in time to save both men caused the First Doctor to finally understand what being "the Doctor of War" meant. Shortly before the Twelfth Doctor left to regenerate, Archibald noticed him standing on the field and confusedly returned a salute. (TV: Twice Upon a Time)

Archibald visited his brother, Alistair 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)

Alternate 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 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.