[caption id="attachment_142985" align="aligncenter" width="1280"] 'The Waters of Mars.' (Pic: BBC)[/caption]

"The Waters of Mars" is a hugely important episode in the history of Doctor Who: The middle part of a trilogy of specials broadcast in 2009 as the final stories featuring David Tennant as the Tenth Doctor (bookended by "Planet of the Dead" and "The End of Time"), it portrays a rare occasion on which the Doctor’s desire to save lives is spurred on by arrogance, and which is ultimately proven to be the wrong course of action. The complex moral dilemma takes place during a story that is also one of the scariest broadcast since the show returned in 2005. "The Waters of Mars" features actress Lindsay Duncan in the role of Adelaide Brooke, the Doctor’s one-off "companion" for this story. (Get a full recap of "The Waters of Mars.")

The episode airs this Saturday, August 15 on BBC AMERICA as part of the special Doctor’s Finest selection of Doctor Who stories—so ahead of that broadcast, here are 10 facts about the story you may not know.

1. The episode was originally intended to be the 2009 Christmas special, and writers Russell T. Davies and Phil Ford hence conceived that "Christmas on Mars" would be the story’s conceit (it was also nearly titled both that and "Red Christmas.") After email discussions with journalist Ben Cook, documented in the book The Writer’s Tale, Davies eventually suggested to the BBC that the two-part finale "The End of Time" be brought forward to Christmas and New Year, and so "The Waters of Mars" was moved to November 2009 as a result.

[caption id="attachment_142986" align="aligncenter" width="1280"] Captain Adelaide Brooke (Lindsay Duncan). (Pic: BBC)[/caption]

2. In another email in The Writer's Tale, Davies mentions going to the cinema and watching three films back to back while struggling with a logistical problem over the third season of Torchwood. One of the films was Disney/Pixar's WALL-E, and a short while afterwards he was inspired by it to include a small robot in the Mars story. As such, Gadget was born.

3. The scenes in the Mars base’s "bio-dome" were filmed at the National Botanic Garden of Wales. The production crew were surprised to discover that upon turning the lights on, hundreds of birds woke up and began chirping, creating a noise that was impossible to edit out of the episode. As a result, Davies quickly wrote additional dialogue in which the Doctor mentioned the presence of the birds, and a shot of a robin was added to the episode.

[caption id="attachment_142987" align="aligncenter" width="1280"] Adelaide and the Tenth Doctor (David Tennant). (Pic: BBC)[/caption]

4. Bowie Base One is named after the singer/songwriter David Bowie, and is an obvious allusion to his 1971 song Life on Mars?—which also, of course, inspired the TV series of the same name.

5. The orange spacesuit the Doctor wears in the episode is the same one he first picked up from Sanctuary Base 6 in the 2006 story "The Impossible Planet"/"The Satan Pit." Similar-looking suits have also been worn by him in "42," "Hide" and "Kill the Moon"—although in all three cases, those suits are missing the SB6 logo patch, so may not be the exact same outfit.

[caption id="attachment_142988" align="aligncenter" width="1280"] The Time Lord Victorious. (Pic: BBC)[/caption]

6. In the initial drafts of the script, Adelaide Brooke survived the episode after being rescued by the Doctor. As explained by Davies in The Writer's Tale, however, it became apparent to him that her death was necessary to drive home to the Doctor the true cost of his arrogance – and so it was decided that she would kill herself, thus restoring the historical timeline. This decision required careful consideration of how the act could be sensitively depicted, and Davies resolved to ensure that Adelaide’s laser gun would not be shown to be firing at any point in the episode, so as not to create too graphic an image in viewers' minds when her offscreen suicide occurred at the end.

7. Another aspect of the episode that created a tonal headache for the producers, as documented in Doctor Who Confidential, was the water monsters. Initial concept drawings for the creatures were considered to be far too scary for Doctor Who’s intended audience, and the crew were close to reconsidering the concept entirely before artist Neill Gorton produced a more suitable version. There were also practical difficulties realizing the monsters: Early on in the process, it was discovered that the planned effect of water dripping down the creatures’ faces was making it impossible for the actors to deliver their lines, due to the water constantly getting into their eyes. The masks were hence reconfigured so that water only emerged around the mouth area.

[caption id="attachment_142989" align="aligncenter" width="1280"] One of the terrifying water monsters. (Pic: BBC)[/caption]

8. The episode flashes back to events set during the fourth season episode "The Stolen Earth," as Adelaide recalls her encounter with a Dalek. The Doctor refers to this event as happening fifty years prior to the date in "The Waters of Mars" of 2059. Although "The Stolen Earth" was broadcast in 2008, the entire "present day" timeline of Doctor Who has been one year ahead of reality ever since the time jump at the beginning of 2005's "Aliens of London."

9. When the Doctor is agonizing over whether to save the crew, he remembers previous dialogue snatches about being "the Last of the Time Lords" from the episodes "Gridlock," "The Doctor’s Daughter," "Doomsday," "Rise of the Cybermen" and "Utopia."

[caption id="attachment_142990" align="aligncenter" width="1280"] The Tenth Doctor in 'The Waters of Mars.' (Pic: BBC)[/caption]

10. The episode won the 2010 Hugo Award for Best Dramatic Presentation, Short Form. This was the fourth time Doctor Who had won the award, having also done so in 2006, 2007 and 2008; further wins would follow in 2011 and 2012.

NEXT: 10 Things You May Not Know About ‘The End of Time’

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