Oo, now that was a good episode! It drew together a lot of the sub-plots that have been in series 5 so far. It was also proper scifi in its sense of scale and vision.
Basically, Vincent Van Gogh did one last painting before he died, a painting to serve as a warning to the Doctor in the future. The painting is discovered and successive generations carry the message forward until it gets to Dr River Song in the year 5145 (remember her from the Angels episode?). Anyway, River then travels back in time to ‘Planet One’, the first planet in the universe, and scrawls a message on the 50ft cliffs for the Doctor. She tells him to meet her at a certain place and time in history – Roman Britain, near Stonehenge.
The Doctor duly turns up in Roman Britain and he and River realise that Stonehenge is a giant transmitter sending a message into space. They go beneath the henge (to the Underhenge) and discover the Pandorica Box, which apparently serves as the prison to the most dangerous being in the universe. The Daleks, Cybermen, Silurians and all manner of baddies (who have formed the Alliance) turn up, apparently to free the monster in the box, and the Doctor has to stop them.
I won’t spoil it by giving away the ending, but it’s great. Easily the best episode of the series so far, although it doesn’t excuse some of the weaker ones we’ve had to endure. Well done BBC. And Matt Smith performs well too.
Leaving only the question of Amy. Well, a few clues. Amy’s favourite books when she was young were Pandora’s Box and a book about Roman Britain. When she was a girl, she made a doll of the Doctor (based on an early encounter with him because of a crack in time that appeared in her bedroom wall) and a model of the tardis. All the elements of this episode come from the mindscape of Amy Pond, therefore. Cosmic and metaphysical stuff!