As well as being a tense parable, partly about the ethics of colonialism, "42" contains a couple of interesting firsts for Doctor Who. It's the first story that attempts to show events happening in real time from start to finish (more or less), it's the first Doctor Who story named after a number and it has the shortest name of any Doctor Who story. And it's the first script written for the show by Chris Chibnall, soon to take on the mantle of showrunner from Steven Moffat. It also shows the Doctor acting in an uncharacteristically emotional manner, appearing helpless, in extreme pain and even terrified.

Here are a few things that you should keep an eye out for, the next time you watch.

(The episode is available on iTunes and Amazon.)

Chris Chibnall's original plan was to place all the action on a space station that had been in use for generations, the crew having been there since birth. Russell T Davies realised that there had already been quite a few stories that took place on space stations, but very few on a single spaceship.

The design of the masks—worn by the crew to protect them from the glare of the sun (but ultimately worn by those crewmembers who were possessed by the spirit of the sun from within—was based on the Marvel Comics character Cyclops, who wore a ruby visor over his eyes to prevent people from being hurt by the blast of his optic rays. He opens the visor with a button at his temple.

Some of the character names were changed during scripting, as is fairly common practise in this sort of thing. Ashton was originally called Motta, but there was a worry audiences would confuse that with Martha during emotional scenes. And Riley Kincade becoming Riley Vashtee when it turned out that Brannigan from "Gridlock" had Kincade for a middle name.

The name of the spaceship had to be changed from the original script, as Chris Chibnall had gone with a reference to ancient Greek myth and called it the S.S. Icarus, after the myth of the man who had been built wings and could fly, but ignored advice not to get too close to the sun, and fell to his death. During production it was noted that the movie Sunshine also featured a spaceship called the Icarus II, also hurtling towards a sun, so the ship was renamed S.S. Pentallian, after the pentallion drive, part of a transmat used in the Fourth Doctor story "Revenge of the Cybermen."

There is a precedent for the Doctor being able to survive exposure to extreme temperatures. The First Doctor endured being frozen to 200 degrees below zero and placed on a plinth in "The Space Museum". Granted, he wasn't also being possessed by the furious spirit of a sun at the time, but needs must.

If you're a fan of props and scenery, you'll no doubt have spotted that the stasis chamber aboard the Pentallian was originally built to be the hospital MRI scanner in "Smith and Jones". And of course the Doctor's orange spacesuit—last seen in "The Impossible Planet" and "The Satan Pit" and due for many return journeys—makes a welcome return.

After last week's visit from Coronation Street soap royalty, a representative of EastEnders—Britain's other biggest soap opera—appears in this episode. Michelle Collins, who plays Kath McDonnell, is a star of one of the soaps biggest and most beloved storylines, playing the flawed and manipulative Cindy Beale. She is also one of the few British actors to make the jump from EastEnders to Coronation Street.

The Doctor attempts to rouse the crew to action by yelling "where's your Dunkirk spirit?", a reference with which British viewers will have been very familiar. The battle of Dunkirk took place from 26 May – 4 June 1940, and was a pivotal moment for British (and French) forces, who, in the face of severe German military might, were forced to evacuate mainland Europe and retreat to the British Isles. The "Dunkirk spirit" is a reference both to finding strength in your darkest hour, and also to the fact that, against all odds, 330,000 troops safely made the journey across the English Channel.

Incidentally, it was recently announced that Tom Hardy, Cillian Murphy and Mark Rylance will be appearing in a Christopher Nolan movie about Dunkirk, coming theaters in 2017:

https://www.youtube.com/watch?v=rJePvN_4T_E

The answer to the question about No.1 records is actually more complex than it appears. If you go by the American stats (according to the various Billboard charts), the Beatles had the greater amount of chart-toppers, having secured 20 to Elvis's 17. However, the UK charts are a different affair, with Elvis having managed 18 to the Beatles' 17. Of course, if you add both together, the Beatles come out on top.

The title of the story has three potential meanings. There's the countdown within the tale itself, a useful amount of time in that it is broadly the length of one episode. Then there's a hat-tip to the TV show 24 (which also depicts events as they would occur in real time) and a more general tribute to Douglas Adams, the former Doctor Who scriptwriter and creator of The Hitchhikers Guide to the Galaxy, in which the number 42 plays a pivotal role (no spoilers!).

More Douglas Adams references from Doctor Who (and popular culture in general).

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