"Turn Left" is by some stretch the darkest story in the whole of Doctor Who's fourth season. It might seem spoilery to mention that both of the two main characters die — although this is not the blog post to read if you wanted to avoid that sort of thing — but that's not even the worst of it.

All of the Doctor's friends die too, apart from Captain Jack who is captured and sent to Sontar. Every good thing the Doctor did from the moment he met Donna Noble has been undone, and Donna herself has no idea of her true potential. It started out as a test by Russell T Davies to see whether the interference of the Doctor actually saves lives or makes things worse, and ends up with Britain being half destroyed by a falling spaceship and electing a fascistic government. Oh and by the end, all of the stars in the universe start winking out of existence.

Thankfully, that's not how it ends. Here are a few things to keep an eye out for, the next time you watch:

(The episode is available on Amazon and iTunes.)

Russell T Davies conceived of the idea for this "Doctor-lite" story before Catherine Tate had been approached to resume her role as Donna Noble from "The Impossible Bride." He had come up with an alternative companion called Penny, whose first adventure with the Doctor would involve a huge dome being placed over London. She would be seen choosing to turn left, entering the area under which the dome would be placed, and thereby find herself meeting the Doctor. Then in the alternate version of this story, she'd be seen turning right, being on the outside of the dome as the Doctor dies. Once Donna Noble had been brought back to the story, it gave Russell a chance to revisit two seasons of Doctor Who stories to see what would happen if the Doctor had never been there.

One of the stories he considered revisiting was "The Shakespeare Code," thanks to a squad of UNIT "time commandos," who were to go back and try and fend off the Carrionites without the Doctor's expert help. However appealing an idea, this would've taken up valuable time in a story already so packed with significant events that its broadcast time was extended to 50 minutes, five minutes longer than the other stories in Season Four.

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

Although David Tennant appears at the beginning and end of the story, the person under the blanket on the stretcher, the owner of the hand that drops the sonic screwdriver as Donna watches the immediate aftermath of the Doctor's defeat of the Racnoss, is not him. It's a body double instead.

The Time Beetle on Donna's back is a deliberate nod to the Eight Legs, a species of giant spider with psychic powers. They could also influence human behavior, mounted themselves on the backs of their human victims, and could disappear at will:

https://www.youtube.com/watch?v=8VvZ-VaeB5g

The Doctor describes the Time Beetle as being one of the "Trickster's brigade," which is a reference to a villain familiar to viewers of The Sarah Jane Adventures. His principal form of attack is to try and rewrite history, using time travel.

In the news report of her death, Sarah Jane Smith is referred to as "a freelance investigative journalist, formerly of Metropolitan Magazine." The Metropolitan, which is seldom mentioned on screen during Sarah Jane's time in the TARDIS, appears more frequently in Doctor Who books, audio adventures and comics — such as "System Shock," "Comeback" and "The White Wolf." It's described as a weekly news magazine, which launched in 1967, although by the time of the comic adventure "Pirates of Vourakis," which takes place in the future, it has been rebranded as Transmetropolitan.

This isn't the first time a mirror has been used in the pursuit of time travel. Victorian inventors Edward Waterfield and Theodore Maxtible build a time machine from mirrors and static electricity, but are horrified when the Daleks arrive to force them to set a trap to ensnare the Second Doctor, in "The Evil of the Daleks." And that ring of mirrors took on a different use entirely when used to defeat the Mara — just as it was used to reveal the Time Beetle — in the Fifth Doctor's jungle adventure, "Kinda."

https://www.youtube.com/watch?v=3yE7e0jEjDM

There's a passing reference by Donna to the time her slightly psychic friend Alice Coltrane, who can see the Time Beetle on her back, claimed to have seen "the ghost of Earl Mountbatten at the boat show." This is a reference to Louis Mountbatten, 1st Earl Mountbatten of Burma, uncle of Prince Philip, who was killed in 1979 when a bomb placed by the Provisional Irish Republican Army (IRA) exploded in his fishing boat, during a visit to County Sligo, Ireland.

When Donna finally gets to see inside the alternate reality TARDIS, it's dark and burned out. The original plan was for viewers to see the console room on fire, but this had to be shelved for budgetary reasons.

https://www.youtube.com/watch?v=8kQVtWuvsUU

When Donna is being fired by Jival Chowdry, she sarcastically responds to the news that Martha Jones's hospital has returned from the moon with an exasperated cry of, "Well, isn't that wizard!" Wizard as an adjective is an archaic form of early 20th century British slang that simply means wonderful or splendid. Closely associated with the energetic gentry of the 1920s and 30s, it's the kind of term British viewers will have been familiar with from extensive use in, for example, Enid Blyton's Famous Five series of children's books. But years of mockery have reduced it to sardonic use as the kind of hurrah only posh people would use. Donna could just as easily have used spiffing, top hole or tickety-boo for the same effect.

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