The read through for The Pandorica Opens was set to commence at 16.00 on Wednesday, 13 January 2010 at Upper Boat. It began later than planned as heavy snow and near freezing temperatures delayed many people's arrival.

We've seen the Doctor on horseback before, most recently in The Girl in the Fireplace, also written by Steven Moffat.

The Pandorica Opens is director Toby Haynes' Doctor Who debut. His previous credits include Being Human, written by Toby Whithouse, who also penned School Reunion and Vampires of Venice.

This is the first episode of the season that doesn't have a throw-forward (mini-trailer) for the following week's adventure, immediately after the action ends. The adventure in question is The Big Bang and is due to premier on the date that has been ominously mentioned since The Eleventh Hour: 26 June, 2010.

River Song travels through time using a Time Vortex Manipulator. We've seen such a device before - Time Agents are issued with them and Captain Jack used one - in fact we first glimpsed a Vortex Manipulator in the Ninth Doctor story, The Empty Child/The Doctor Dances, another adventure written by Steven Moffat.

History and henges

Stonehenge, which features in The Pandorica Opens, is located in Wiltshire, Southern England. Henges are a uniquely British and Irish phenomenon. They consist of a circular ditch and an external bank, usually surrounding settings of posts or standing stones. Stonehenge is the most famous henge in history and although nobody knows exactly why it was built or when, we do know that the first stone circles were erected around 3300 BC - 1200 BC. Find out more about Stonehenge.

The date of this year's Summer Solstice is 21 June - just two days after the premier of The Pandorica Opens. Stonehenge is closely associated with the Solstice and although the site of the stones is now carefully controlled, at one time many people gathered at the henge to mark the event.

River Song pretends to be Cleopatra, or more precisely, Cleopatra VII who ruled Egypt from 51 BC - 30 BC. She is remembered chiefly for her beauty and tempestuous relationships with the Roman warlords Mark Antony and Julius Caesar. Shakespeare immortalised her in Anthony and Cleopatra and in The Girl in the Fireplace, Mickey Smith suggests she was a friend of the Doctor's. He doesn't confirm this but in The Masque of Mandragora he did mention that one of her soldiers taught him swordsmanship. Read more about Cleopatra.

The Doctor mentions the various Roman invasions of Britain. You can take a 3-D virtual tour of a Roman fort or read an overview of the island under Roman rule. Younger viewers can find out all about Roman Britain with timelines, a game and info on the soldiers and rebellions.

Myths and Monsters

According to some myths, most notably the tales of the 12th Century story-teller Geoffrey of Monmouth, Stonehenge is the work of the magician, Merlin, who is said to have were gathered the stones by sorcery and constructed the site using his supernatural powers. In the Seventh Doctor adventure, Battlefield, it's suggested that the tales of Merlin are actually based on the Doctor's exploits and that he was and is Merlin!

The Autons are back! The Doctor has previously encountered the Autons in Spearhead from Space (1970), The Terror of the Autons (1971) and Rose (2005). They are the humanoid plastic servants of the Nestene Consciousness; some have blank, featureless faces whereas others so closely resemble humans that they are virtually indistinguishable from people. River detected a Nestene presence when scanning the gathering enemies, giving a clue as to what lay ahead.

When River reeled off the list of aliens present in the skies above Stonehenge, some, such as the Daleks, Cybermen, Slitheen, Nestene, Sycorax and Atraxi are familiar from (relatively) recent stories. However, some of the other gathering foes haven't been seen for many years. The Doctor last faced the Zygons in the 1975 story, Terror of the Zygons. They are a brutal race with the ability to change their shape. The Loch Ness Monster is in fact one of their cyborgs and it took the Fourth Doctor, UNIT and Sarah Jane to prevent their plan to take over Earth after their home planet was destroyed by solar flares.

Drahvins are also mentioned; they last appeared in the 1965 story, Galaxy 4. A predominantly female species they were beautiful but deadly and the Doctor found them warlike and pitiless.

The Draconians, also lurking in the skies over the henge, were encountered by the Third Doctor in The Frontier in Space. They are proud, reptilian creatures from the planet Draconia. Although the Doctor was able to cement a peace between Draconians and humanity, a war between the two races once raged and hostility between them lingered long after the last shot was fired.

River mentions the presence of Chelonians although they have never appeared onscreen. In an early version of the plot for the 2009's Easter special, the Doctor was to have encountered them. However, the story took a different direction and eventually became Planet of the Dead - without the Chelonians.