[caption id="attachment_143137" align="aligncenter" width="1280"] 'The End of Time' (Photo: BBC)[/caption]

"The End of Time" is more than just a regeneration epic, a dabble in Time Lord politics and an unmasking of the madness behind the Master. It is the final rounding off of the first part of the modern Doctor Who era, while still scattering a few seeds here and there for events that will take a few more years to fully germinate and grow (such as the Moment, the Rose Tyler-shaped bomb in "The Day of the Doctor"). It's also the first Doctor Who story that really shows the Doctor as feeling old within his own regeneration and yet conflicted about having to make the change. Not for nothing is the Tenth Doctor's "I don't want to go" regarded as one of the most emotional scenes in the show's history.

The episode airs this Saturday, August 22 on BBC AMERICA as part of the special Doctor’s Finest selection of Doctor Who stories (and there's a full recap of "The End of Time" here), so before we get stuck in, here are 10 facts about the story you may not know.

1. Time is not the only thing that may be ending, as we say a final farewell to the Tenth Doctor, Donna Noble, Wilfred Mott, Martha Jones, Mickey Smith, Captain Jack Harkness, Sarah Jane Smith (in Doctor Who at least, The Sarah Jane Adventures continued until 2011), and another regeneration of the Master, the one before Missy arrived. It was also the final story for execs Russell T Davies and Julie Gardner, with the gauntlet being handed over to Steven Moffat for Matt Smith's regeneration scene. The only comparable total changeover in Who history was between the Second and Third Doctors ("The War Games"/"Spearhead From Space") and the arrival of the Ninth Doctor ("Rose") when the modern show began in 2005.

[caption id="attachment_143143" align="aligncenter" width="1280"] Timothy Dalton as Rassilon in 'The End of Time' (Photo: BBC)[/caption]

2. Timothy Dalton's appearance as the spittle-raging Rassilon is a throwback to classic Who and a puzzle for long-term fans. How could the man famed as the founder of Gallifreyan civilization—the man who harnessed a super-nova, developed time travel and became first Lord President of Gallifrey—still be in charge during the Time War? Well the clue is in the title of the conflict. It's a time war; history is an active combatant. We already know that, for Gallifreyans, death is not always the end. And there had been references to the dead coming back from their graves, and the Master had already said he had been "resurrected" in "The Sound of Drums," implying that things were happening beyond the relatively simple regeneration cycle. In any case, as he appears to hold the key to immortality in "The Five Doctors," it's possible Rassilon can't (or simply refuses to) die.

3. The reference to Geoffrey Noble, from whom the Doctor borrows a pound to buy a lottery ticket, is a tribute to the actor Howard Attfield. Howard had played the part of Donna's father in her first story, "The Runaway Bride," but passed away having shot some of the scenes for her return, "Partners in Crime." His essential role in proceedings was taken up by Wilfred Mott—played by Bernard Cribbins—a character that had only appeared in "Voyage of the Damned" as a one-off, but once he was established as Donna's grandfather, he quickly became not only a fan favorite, but a perfect foil for a discussion of the Doctor's pre-regeneration woes.

[caption id="attachment_143140" align="aligncenter" width="1280"] Claire Bloom and Bernard Cribbins in 'The End of Time' (Photo: BBC)[/caption]

4. The identity of the woman who appeared to Wilf early in the tale and exchanged significant looks with the Doctor over Rassilon's shoulder (as played by Claire Bloom) was kept deliberately vague. In The Writer's Tale, his insider view of the creative process, Russell T Davies wrote that he didn't want anyone to be too sure of who she was, although he had a pretty strong inkling himself: "I like leaving it open because then you can imagine what you want. I think the fans will say it's Romana. Or even the Rani. Some might say that it's Susan's mother, I suppose. But of course it's meant to be the Doctor's mother.

"It could only be his mother, really. If I can't imagine a world in which our mothers are there, at the end of our lives, in our time of need, to help us, then what's the point? It'll never really happen, so I want to imagine it."

5. David Tennant had hurt his back and had to have an operation while performing Hamlet at the Royal Shakespeare Company, which made that stunt where he was strapped to a strange upright gurney and wheeled down a flight of stairs a bit dicey (although it took many, many takes). So they used a painstakingly accurate David Tennant dummy that must surely be in a store-room somewhere, gathering dust. Compelling thought, eh?

6. The Eighth Doctor claimed to be half human on his mother's side in the 1996 Doctor Who TV movie, a claim which has either been put down to an error in scripting or the Doctor making a jokey reference to the Star Trek character Mr. Spock, who genuinely is half human on his mother's side. Russell T Davies addressed this in a scene (deleted at the scripting stage) set aboard the spaceship Hesperus, in which the spiky-topped Adams and Rossiter demand to know if their new guest is human, and the Doctor responds, "Nope. Well, I was, back in 1999 for a couple of days, but that was like catching a 48-hour bug, I got over it."

[caption id="attachment_143139" align="aligncenter" width="1280"] David Tennant as the Doctor in 'The End of Time' (Photo: BBC)[/caption]

7. When Wilf is discussing his military service with the Doctor, Russell based it on the details of Bernard Cribbins' National Service in the Parachute Regiment, which took place in Palestine. The blizzard Wilf refers to was actually snow, according to Bernard, but Russell altered the wording to make it more ambiguous, lest anyone accuse him of making up impossible climate conditions for sentimental reasons.

8. The multi-Master cliffhanger at the end of part 1 was one of the costlier effects in an already very expensive shoot, so at one point it was considered that he would just take over the minds of everyone on Earth (apart from the Doctor, Wilf and Donna) rather than their faces too. This didn't appeal to Russell as a solution, and he briefly considered offering to pay for the effect out of his own pocket. Thankfully "Planet of the Dead" came in under budget, which may just have helped to save the day.

[caption id="attachment_143138" align="aligncenter" width="1280"] John Simm as the Master in 'The End of Time' (Photo: BBC)[/caption]

9. Jessica Hynes makes a return, playing the granddaughter of Joan Redfern, who the Doctor fell in love with when he was the human John Smith in "Family of Blood"/"Human Nature." Her character's name is Verity Newman, a name derived from Verity Lambert and Sydney Newman, the producers who put Doctor Who together in 1963. See An Adventure in Space and Time for more details.

10. This isn't the only adventure of the Doctor's that bears the title "The End of Time." There's a novel with the same name, written by Justin Richards and released in September 2009, three months before the TV episodes aired. It's the concluding part of the 10-book Darksmith Legacy series and has no relationship with the TV story whatsoever

NEXT: 10 Things You May Not Know About ‘The Eleventh Hour’

Now read the rest of the 10 Things You May Not Know About Doctor Who archive.