"The Pandorica Opens" is a story which contains a huge amount of exciting and provocative ideas for long-term fans. There's the jumps in time and space from the remote beginnings of the universe to Roman Stonehenge and on to the present day. There are the legions of alien races who have lined up against the Doctor, to spring a particularly well envisioned trap, and there's the possibility that one man's reputation can be enough to ward off an invasion.

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

(The episode is available on iTunes and Amazon.)

The alien races that have united against the Doctor in this episode span not only the modern TV show - Daleks, Nestenes, Cybermen, Sontarans, Atraxi, Slitheen, Silurians, Sycorax, Judoon, Hoix - and the classic series - Zygons, Drahvins, Tereleptils, Draconians - but also Torchwood - Weevils, Blowfish - The Sarah Jane Adventures - the Uvodni - and even the Haemogoths from Brian Minchin's novel The Forgotten Army and the Chelonians from Gareth Roberts' Doctor Who novel (and audio play) The Highest Science.

Apart from saying "HELLO SWEETIE," the message left for the Doctor by River Song starts with "ΘΣ Φ ΓΥΔϟ," the first two symbols of which are Theta Sigma. In the Fourth Doctor story "The Armageddon Factor," it is revealed that Theta Sigma was the Doctor's nickname at the Time Lord Academy on Gallifrey.

The idea of a cliff face with a significant historical message etched onto it has echoes within Douglas Adams' So Long and Thanks for All the Fish, part of The Hitchhikers Guide to the Galaxy series. His version is the last message from God to his creation, on the planet Crafe Tec Heydra: "One side of a mountain carries carvings and hieroglyphs, crude representations of an invisible War. The artwork shows two races clashing, one metal, one flesh; a fearsome explosion; and a solitary survivor walking from the wreckage. Solitary? Perhaps not. Under this figure, a phrase has been scratched in the stone, which translates as: 'you are not alone.'"

And while we're on fandom crossovers, Simon Fisher-Becker plays the blue-skinned Dorium. His movie resume also includes a stint as the Fat Friar, a ghost in Harry Potter and The Sorcerer's Stone.

While this is easily the most highly regarded example, the Doctor has a habit of yelling "I AM TALKING!" when there's a lot of hubbub happening in his presence. The Ninth Doctor said it to the Nestene Consciousness in "Rose" and the Tenth said it to Eddie Connolly in "The Idiot's Lantern."

https://www.youtube.com/watch?v=Pa74e8oAvIM

If you're wondering why Stonehenge is lit by electric lights, while the Doctor, Amy and River explore the caverns around the Pandorica using flambeaux torches, it's deliberate. That part of the story was designed to evoke a similar atmosphere to that of the Indiana Jones movies. Director Toby Haynes even played some of the music while filming the "under Henge" sequence, to get the pacing right.

The Doctor, Amy and River are shown riding horses to Stonehenge, a stunt which necessitated them sitting on saddles on the back of a truck and pretending to ride while the truck drove. Their wide shots were of real horses, but fake (i.e. stunt) actors.

River's costume was designed to evoke two major characters within the Star Wars universe: Princess Leia and Han Solo.

At the time of broadcast, the entire universe had been threatened four times in Doctor Who history. By the Sontarans in "The Invasion of Time," by the Master in "Logopolis," by Davros and the Daleks in "Journey's End," and by the Time Lords in "The End of Time."

For the scenes in the Underhenge where Amy is attacked by the one-armed Cyberman, the crew hired amputee actor Tim Baggaley. But during the edit, the production team decided to reshoot that section. However, Tim was not available for the reshoot, so veteran monster-suit artist Jon Davey was drafted in, with his arm (and head) covered in green screen material:

https://twitter.com/jondavey007/status/799151731263291392

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