"The Impossible Astronaut" sets a high bar for a Doctor Who season opener, not least by shooting the show's title character and then giving him a viking burial. It's also the first Doctor Who serial to have been filmed on American soil (although not the first to have been set there) and introduces the importance of the roadside diner to the future history of the show.

Here are a few things to keep an eye out for, the next time you watch (warning: some spoilers):

(The episode is available on iTunes and Amazon.)

The Doctor makes the TARDIS invisible (with a little help from River) and lands it silently. Both of these things have been done before in the classic TV series. The invisible TARDIS made an appearance in the Second Doctor adventure "The Invasion" while the silent landing happened earlier, as piloted by the First Doctor in "The Dalek Invasion of Earth."

The Doctor says, "Brave heart, Canton," a nodding echo of his most reassuring catchphrase during his Fifth incarnation: "Brave heart, Tegan."

When River slapped the Doctor, the moment of slapping had to be shot several times, and to make it look and feel real, Alex Kingston had to genuinely slap Matt Smith. Needless to say, the appeal of being slapped several times soon wore off for Matt, not least because his cheeks were becoming red and rather raw.

This isn't the first time Stuart Milligan, who played President Nixon, has appeared in the Whoniverse. He also appeared in the animated Tenth Doctor special Dreamland as the voice of Colonel Stark, and popped up in The Sarah Jane Adventures story "Warriors of Kudlak," playing Tannoy.

Sydney Wade, who played the child in the spacesuit we would later discover to be the young River Song, had just finished shooting Marchlands when she arrived at Doctor Who. In a neat moment of synchronicity, she played the daughter of Kingston's character.

President Nixon's security guards claim that the Doctor presents a "clear and present danger" to his life. The term, which achieved popularity as a test of the limits of the First Amendment of the U.S. Constitution, originates in U.S. legal language of the early 1900s. It was used as a test for when a citizen could be prosecuted for their speech — suspending their First Amendment rights — if what they said could be proven to be harmful to public interest. Justice Holmes used the phrase in this summing up of Schenck v. United States, a 1919 case in which an antiwar activist claimed the right to free speech for advocating resistance to the draft:

"The question in every case is whether the words used are used in such circumstances and are of such a nature as to create a clear and present danger that they will bring about the substantive evils that the United States Congress has a right to prevent."

The Doctor claims the destination of the Ponds and River Song is "space, 1969," which is a nod to the title of the 1970s British science fiction TV series Space: 1999.

Mark Sheppard, who played Canton Delaware, is well known to science fiction fans from his various roles in Firefly, Star Trek: Voyager, Supernatural, Battlestar Galactica and Warehouse 13. Despite being from England, this is actually his first role for a British TV series.

During the opening sequence, the Doctor appears in a clip from the Laurel and Hardy movie "The Flying Deuces," dancing about and wearing a fez. It's not just an example of the Doctor's ability to insert himself into Earth's popular culture, but a direct reference to a skit in the 1992 Academy Awards ceremony, where Billy Crystal was inserted into the exact same clip. Here's the original, unsullied by time-travelers or comedians from the future:

https://www.youtube.com/watch?v=l9hmO-wOx-s

This episode was broadcast just four days after Elisabeth Sladen passed away. As she had played Doctor Who's most popular companion Sarah Jane Smith, the only one of the Doctor's various friends to make a return journey to the relaunched show (and get her own spin-off series too), the episode was dedicated to her, with a special mention before broadcast (in the U.K. and Canada) and afterwards (on BBC AMERICA).

NEXT: 10 Things You May Not Know About ‘Day of the Moon’

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