Test yourself! Take this Twice Upon a Time Quiz | The Series (2005-)

Answer: David Bradley

David Bradley comes back to the role of the First Doctor, in a way, as he played actor William Hartnell in the 2013 television film "An Adventure in Space and Time" which documented the creation of "Doctor Who." Bradley started acting in 1971, and has been a regular on British television since the 1980s.
Bradley is known to Harry Potter fans as Argus Filch. Other recent Science Fiction fantasy appearances include "The Strain" (2014-17) and "Game of Thrones" (2013-17).
Ironically, Bradley is almost 20 years older than Hartnell was in the 1960s, so Peter Capaldi was technically correct when he said "I am younger!"

Richard Hurndall (1910-1984) played the First Doctor in the 20th anniversary special "The Five Doctors". It was one of his last roles.

William Hartnell (1908-1975) created the role of "The Doctor" when the series began in 1963. He appeared in 137 episodes between November 1963 and October 1966. Hartnell can be seen briefly in clips from "The Tenth Planet" shown in "Twice Upon A Time", which was as noted, 709 episodes previous.

Peter Cushing did play "Doctor Who" in two feature films in the 1960s. Cushing was a veteran of British horror and science fiction, including "Star Wars" (1977).

Answer: The South Pole

They meet at the South Pole, which was the setting for most of "The Tenth Planet" the story in which William Hartnell regenerated. The Tenth Planet referred to Mondas, the home world of the Cybermen. The Cyberman played an important role in "The Doctor Falls" and are the ultimate reason for the Twelfth Doctor's need to regenerate.

Trenzalore was the site where the Eleventh Doctor became the Twelfth Doctor in "The Time of the Doctor" (2013). Gallifrey is the home world of the Time Lords.

Answer: Ypres 1914

The Captain was in Ypres, Belgium on December 24, 1914. This would prove significant as it was the time of the Christmas Armistice, an unofficial cessation of hostilities between allied and German forces. Soldiers began by singing Christmas carols and would later exchange souvenirs and even play soccer.

The Marne and The Somme were two of the bloodiest battles in human history. Gallipoli was the site of the suicidal charge by Australian-New Zealand forces.

Answer: Testimony

The group of glass people are part of an organization called Testimony. It uses time travel techniques from the year 5 billion and memory harvesting to create synthetic replicants of the departed. The name of the scientist was Helen Clay of the University of New Earth. The glass woman was voiced by Nikki Amuka-Bird, who has been working in British television since 1998

Since Bill Potts had died (shot in the chest, very nasty), her memories had been stored by Testimony. It allowed Bill to play a major role in "Twice Upon a Time".

Answer: Rusty the Dalek

Rusty the "good" dalek, who we first saw in "Into the Dalek" shows up again as the guardian of a weapons base. The Doctor uses Rusty to discover who the glass woman is by using the Dalek hivemind.

Missy was the last (more likely most recent) version of The Master, who was last seen in "The Doctor Falls". Rigsy the graffiti artist can be seen in "Flatline" and "Face the Raven". The Skovox Blizter (from "The Caretaker") is presumably still floating around in space.

Answer: Lethbridge-Stewart

Mark Gatiss plays Captain Archibald Hamish Lethbridge-Stewart. Since the Captain notes he has sons, and Nicholas Courtney, the actor who played Brigadier Alistair Lethbridge-Stewart was born in 1929, it makes sense that Archibald is the Brigadier's grandfather.
That would make him Kate Stewart's great grandfather.

Brigadier Alistair Gordon Lethbridge Stewart was one of the creators of the Unified Intelligence Task Force (UNIT), a military organization dedicated to investigating extra-terrestrial phenomenon. The Brigadier worked with the Second through Seventh Doctors. It was revealed he had died (as had the real Courtney) in 2011.

Mark Gatiss has been active in the new "Doctor Who" since the beginning, writing "The Unquiet Dead" (2005), the third episode of the new series. Other Gatiss episodes include "The Crimson Horror" (2013) and "Sleep No More" (2015). He also played Doctor Lazarus in "The Lazarus Experiment" (2007). Gattis also wrote "An Adventure in Space and Time," the 2013 TV film that starred David Bradley, bringing things full circle.

Answer: Clara Oswald

The Testimony was able to restore The Twelfth Doctor's memories of Clara, which he had lost during "Hell Bent" (2015). Jenna Coleman makes a brief cameo appearance to chastise the Doctor for forgetting her.

Clara Oswald, AKA "The Impossible Girl" or "Souffle Girl" was a companion of the Eleventh and Twelfth Doctors. She saved every incarnation of The Doctor by jumping into his time stream in "The Name of the Doctor" (2013). After that, things got complicated.

Answer: "I let you go."

After a long, rambling lecture to his future self, the Doctor simply decides to leave his current incarnation with a simple "I let you go."

Among the advice the Twelfth Doctor gives is "never eat pears," "hate is always foolish; love is always wise", "always try and to be nice, never fail to be kind" and the somewhat mysterious information that children can understand the name of The Doctor.

Answer: Oh Brilliant

Jodie Whitaker looks into the console, sees her new face and states "Oh brilliant." She can't say much more as the TARDIS explodes and sends the new Doctor plummeting through space. Very reminiscent of Matt Smith's first scene as the Eleventh Doctor or his fall to Earth in "The Doctor, the Widow and the Wardrobe" (2011).

Jodie Whitaker is probably best known for her work in the police procedural "Broadchurch" (2013-17) where she played opposite Tenth Doctor David Tennant. Whitaker has had an impressive career since her breakout performance in "Venus" (2006), with nominations for multiple acting awards. At age 35, she is 24 years younger than Peter Capaldi. However, she is five months older than Matt Smith.

Answer: Rachel Talalay

Since 2014, Steven Moffat has trusted Rachel Talalay with bringing "Doctor Who" seasons to a close. She helmed "Dark Water/Death in Heaven" (2014), "Heaven Sent/Hell Bent" (2015) and the last three episodes of the 2017 season: "World Enough and Time," "The Doctor Falls" and the Christmas special.

Talalay is the first American to direct an episode of "Doctor Who". She was actually a mathematician and computer programmer before getting into show business. While Talalay started in films, she has worked primarily in television since 1997.

Steven Moffat has been the show-runner of "Doctor Who" since 2010 and is leaving with this episode. He has written more episodes than anybody in history, including "Blink" (2007) and "Listen" (2014).

Pearl Mackie played Bill Potts, the Doctor's companion during Season 10. Bill was the first openly gay companion, and she wound up with a young woman named Heather at the end of "The Doctor Falls." "Doctor Who" was Mackie's first huge break as an actress.

Chris Chibnall will be the new show runner for "Doctor Who" starting in 2018. He has been working as a writer in British television since 2002. As the creator and head writer of "Broadchurch" (2013-17), Chibnall has a close working relationship with the new Doctor, Jodie Whitaker. Chibnall has also penned several episodes of "Doctor Who" including "The Hungry Earth/Cold Blood" (2010) and "Dinosaurs on a Spaceship" (2012).