"Human Nature" is a very different form of Doctor Who story from any that has appeared on television before, with the possible exception of some post-regeneration stories.

The Doctor is present, but also entirely absent. His companion has to try and fend off an alien attack using her fairly limited knowledge of what he might do if he was his usual self, but also has to do this while dealing with both his insistence that he is not the man she thinks he is, and social convention (not to mention racism) in a particularly hidebound and regimented place and time in English history.

And this is only part one, part two is even more intense.

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.)

Both "Human Nature" and "Family of Blood" are adapted by Paul Cornell from his novel Human Nature, released as part of the Virgin New Adventures series of Doctor Who Stories. Several of the key elements are the same in both stories, including the family of shapeshifters (named Aubertides in the book), the list of instructions for his companion to follow, and the lack of a plan should the Doctor's human incarnation fall in love, which he does, with Joan Redfern.

However, there are some key differences too. The original had the Seventh Doctor become human to try and experience the grief felt by his companion Bernice "Benny" Summerfield when her partner dies. To do this he used an Aubertide device, and that's why they came looking for him. In the novel, the device that holds the Doctor's essence is a cricket ball, not a fob watch. And the book John Smith writes about the Doctor is more of a children's story than a dream journal. And then there are the scarecrows, included by Russell T Davies as a source of further terror.

The book has the Doctor believing himself to be Scottish, the Aberdeen-born John Smith, whereas the TV John Smith is upper-crust English all the way. In a neat parallel, while both of the actors playing the respective Seventh and Ninth Doctors actually are Scottish, Sylvester McCoy is from Dunoon and his accent is audible in the way he plays the Doctor, while David Tennant is from Renfrewshire but plays the Doctor (and John Smith) with a home counties English accent.

As if playing the Doctor playing an entirely different person entirely wasn't fiddly enough, David Tennant also had to learn how to waltz, with his co-star Jessica Hynes. He claimed that the footage in Doctor Who Confidential of them struggling was a result of being camera shy. Which of course, he would:

https://www.youtube.com/watch?v=8IPAQln2YfM

John Smith claims that his parents are called Sydney and Verity, which is a reference to Sydney Newman and Verity Lambert, the production team who first came up with Doctor Who (as seen in Mark Gatiss's An Adventure in Space and Time). This tip of the hat is then returned to in "The End of Time," when the Doctor visits Joan Redfern's granddaughter at her book-signing, and we discover she has been given the name Verity Newman.

Although this is the Doctor Who role with which Jessica Hynes has made the deepest impression, it's not her first, She can also be heard as Glory Bee (and Carla) in the 2002 Big Finish Eighth Doctor audio drama "Invaders from Mars," written and directed by Mark Gatiss. It's an all-star affair, with a cast that includes Simon Pegg, Jessica's co-star in the cult comedy Spaced, and Katy Manning, who played the Third Doctor's companion Jo Grant.

Harry Lloyd, who plays Jeremy Baines/Son of Mine, is the great-great-great grandson of Charles Dickens.

Possibly as an unexpected side-effect of the Chameleon Arch, the Doctor, who claimed in "Fear Her" that his artistic skills are poor - "stick men are about my limit" - has developed a good eye and a steady hand in his John Smith incarnation. Certainly good enough to draw most of his beloved companions and hated enemies in the book of his dreams, and capture a likeness of Joan that can win her heart.

One thing John Smith has inherited from the Doctor is a certain ability with a cricket ball, first seen in "Black Orchid," a Fifth Doctor adventure set 10 years after the events in "Human Nature"

https://www.youtube.com/watch?v=D2-5LAe_x_0

There's a running gag from classic Doctor Who, which occurs when Joan asks the Doctor if Gallifrey is in Ireland. In "The Hand of Fear," the Fourth Doctor is asked where he is from, and when he replies Gallifrey, his interrogator says "No, I've not heard of it. Perhaps it's in Ireland."

There's an almost identical exchange between the Fourth Doctor's companion Leela and another inquiring bureaucrat in "The Invisible Enemy."

NEXT: 10 Things You May Not Know About ‘The Family of Blood’

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