In "Smile", the Doctor and Bill travel to a beautiful new world in the far future of mankind, marveling at its wonders and possibilities, and then discover something deeply disturbing at its core.

It has echoes of a few previous Doctor Who adventures - not least "The Robots of Death", "The Ark" and "The Doctor Dances" - but none of them have robots with emoji faces, or mood-sensitive emoji-badges that can get you killed.

Here are a few things to keep an eye out for, the next time you watch:

As befits the brief that Season 10 is a kind of fresh start for Doctor Who, so that new fans can jump on board the TARDIS with Bill, the first three episodes of the series follow a pattern established in the Russell T Davies era. Episode one introduces the Doctor to his companion (or the companion to the regenerated Doctor) on contemporary Earth ("Rose" / "The Christmas Invasion"). Episode two takes the travelers into the far future ("The End of the World" / "New Earth"), and episode three is set in Victorian times ("The Unquiet Dead" / "Tooth and Claw").

The spaceship Erewhon is named after the newly-discovered country in the satirical 1872 Samuel Butler novel Erewhon: or, Over the Range (the name is "nowhere" backwards, sort of). The narrator of the book arrives in a new country, concludes that it is a wondrous place, and then is quickly disabused of this assumption after finding out how society treats criminals, people who are unwell, and the improving aspects of science.

And while we're on names, Frank Cottrell Boyce named the Vardy were named after the Canadian scientist Andrew Vardy, who works with swarm robotics. Frank and Andrew had previously collaborated on a short story, as Andrew explained to CBC NL:

https://www.youtube.com/watch?v=TGyJagthD9I

Bill tries to find out why the Doctor hasn't fixed the TARDIS's chameleon circuit, but in fact it's not from the lack of trying. The Fourth Doctor made an attempt in "Logopolis," but the Master got in the way. And the Fifth Doctor actually got it working for a short time. The TARDIS appeared as an organ, then a wardrobe, then a gate, before returning to it's more common shape. By the time the Ninth Doctor discusses the topic with Jack, Mickey and Rose in "Boomtown," he's more or less given up. Jack asks "why don't you just fix the circuit?" and the Doctor replies, "I like it, don't you?"

Fans of The Sarah Jane Adventures will have already spotted that Mina Anwar (who played Gita Chandra) has finally made the jump into full Doctor Who, as she plays the unfortunate Goodthing. Similarly, Ralf Little, who plays the soldier Steadfast, has graduated to a full Doctor Who story after playing Guy Fawkes in the Doctor Who video game The Gunpowder Plot.

There are a few throwbacks in this story to the Seventh Doctor adventure "The Happiness Patrol." The idea that people need to demonstrate that they are happy or risk being killed was a law on the planet Terra Alpha, put in place by the dictatorial leader Helen A. The relationship between the Seventh Doctor and his companion Ace is also fairly similar to that of the Doctor and Bill. Ace often referred to the Doctor as "Professor" and would react enthusiastically to things by exclaiming "wicked!", which Bill also says, when she discovers the spaceship Erewhon isn't locked.

Although it looks very much like an alien city, the location set for the buildings of Gliese 581d was not actually built by the Vardy. They are part of The City of Arts and Sciences (Ciudad de las Artes y las Ciencias), a cultural and architectural centre in the city of Valencia, Spain.

Bill suggests Aberdeen or Wiltshire as places she would like to visit with the Doctor, but actually the Doctor has been there before. The Third Doctor's run-in with "The Daemons" took place in rural Wiltshire, while the Fourth Doctor's attempt to drop Sarah Jane Smith off in South Croydon (in "The Hand of Fear") went awry, and she ended up having to make her way home from Aberdeen.

There's a tiny David Bowie reference in the section where the Doctor goes back to the colony with the intention of blowing it up. Cornered by an Emojibot, he repeatedly says "I'm happy" before saying "I'm happy, I hope that you're happy too," a slight paraphrase of a line from the song "Ashes to Ashes."

A second musical reference comes a short while later, as the Doctor finds the outer wall of the Erewhon, saying, "A wall, a real, honest wall, not made of tiny robots but made of any old iron." "Any old iron" entered vernacular English from the highly regarded song of the same name by British music hall star Harry Champion in 1911. Warning: this recording is probably quite hard to follow to ears not attuned to a London accent, or slang sentences such as "you look dapper from your napper to your feet":

https://www.youtube.com/watch?v=a4GdWK_WoNs

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