"Planet of the Ood" is the closest thing Doctor Who fans will get to an origin story for one of the most instantly beloved alien races in the new series. Their calm voices and subservient manner all needed further explanation and exploration and this is where it all happens.

It is also another story in which Donna is given the chance to experience reality the way the Doctor does, which helps build their relationship after a few choppy moments in her early travels with him that severely challenge her moral compass.

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.)

Having enjoyed their appearance in "The Impossible Planet" / "The Satan Pit," Russell T Davies wanted to explore something of the history of the Ood in a future episode. At first he considered putting them on the sun-mining ship in "42," then explored the idea of elongating this story into a two-part adventure. Early versions of the script included a long sequence where the Doctor looks for the big Ood brain in a series of underground caverns.

On learning exactly where they have landed, the Doctor says "The Ood-Sphere, I've been to this solar system before. Years ago. Ages. Close to the planet Sense-Sphere." This is a reference to the First Doctor adventure "The Sensorites," in which he and his fellow travelers meet an alien race of similar demeanor and appearance to the Ood, and similarly telepathic abilities:

https://www.youtube.com/watch?v=0P-aj6PB6S4

Writer Keith Temple noted in Doctor Who Magazine that this is one of the rare occasions where a new companion to the Doctor visits an alien planet for the first time and comes away relatively unchanged, because her ideals and morals are what drives her to act in the way she does: "I don't think it's so much a case of how Donna develops as a person during this adventure. It's more about how people in the Ood-Sphere develop as a result of coming into contact with her!"

When the Doctor asks Donna where she learned to whistle she replies, "West Ham, every Saturday." This is, of course, a reference to the East London soccer (or football, to British viewers) team, and the general rowdiness of their fans. The team was originally named the Thames Ironworks Football Club, as an amateur side of workmates from a factory in Canning Town. They rebranded and relaunched as West Ham United in 1900.

One of the actors underneath the Ood masks, appearing as an uncredited extra, was Peter Symonds, who had previously appeared in Doctor Who back in 1975, playing a soldier in the Fourth Doctor adventure "Terror of the Zygons."

The Doctor claims to be delighted to finally experience real snow, as his last three encounters were all atmospheric tricks. First with the Sycorax ship breaking up in the atmosphere in "The Christmas Invasion," then an artificial snowfall the TARDIS created in "The Runaway Bride," and finally the ballast from the spaceship Titanic, in "Voyage of the Damned." Actually even this snow wasn't real, as the episode was filmed during a hot week in August. The snow was actually shredded paper.

Continuing the tradition—from "The Fires of Pompeii"—of bringing in some iconic figures from British comedy TV, Klineman Halpern is played by Tim McInnerney, who British viewers will have been very familiar with as Lord Percy/Captain Darling in the various series of the beloved historical comedy Blackadder:

https://www.youtube.com/watch?v=TkZFuKHXa7w

Halpern's transformation from human to Ood was originally filmed in far more detail, but during the edit it was toned down as it was considered far too horrific for a family audience.

Although that scene was largely filmed using layers of prosthetics, when it came to shoot the CGI section where the Ood tentacles and hind brain emerge from Halpern's mouth, Tim McInerney was unavailable. The production team's best boy Peter Chester had to stand in, recreating the movement of the lower half of his face using motion capture techniques.

In an interview with Doctor Who Magazine, David Tennant revealed that part of the inspiration for delving into the history of the Ood was anthropological: "This is the story behind the service industry. What's going on beneath the tentacles? Why were the Ood so susceptible to suggestion? What makes them so keen to serve, and makes them so selfless? What's that about? Can we really believe that this is all they want to do? Are they too good to be true?... Can there really be a race who just want to help others? And wouldn't that disprove Richard Dawkins' selfish gene argument?"

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