Well. How about that? This is the episode where it all starts happening and there are quite a few surprises along the way.
The start of the episode jumps around through time and manages to reference quite a few of the previous 11 episodes. It's 1890 and Van Gogh lies dying after completing his last painting. Now it's 1941 and Churchill tries to phone the Doctor to warn him of what the paining contains. Now it's 5145 and River Song takes the call. She leaves a message for the Doctor written on the oldest cliff in the universe with time coordinates of where to meet her. Now it's 102 AD and the Doctor and Amy arrive in Roman Britain. (Note, this River has yet to encounter the Weeping Angels from the episode earlier in this series!)
River shows the Doctor the painting of the Tardis exploding. They trace a signal to Stonehenge. Under Stonehenge they discover the Pandorica, a big box that was built as a prison for the most feared being in the universe. It's sending out signals announcing it's opening and the Doctor realises that every alien race will come for it. River confirms that Daleks, Cypermen and just about everyone else have arrived above Earth.
One of the Roman soldiers turns out to be Rory who is as a loss how to explain how he's been resurrected. Amy doesn't remember him but finds herself crying because she is happy.
River goes to fetch the Tardis but it takes off and arrives at Amy's house on 26/06/2010. Inside she finds Amy's childhood books on Roman Britain and Pandora's Box. She warns the Doctor that Amy's memories have been used to create a trap.
The Romans turn out to be the plastic Auton guys and capture the Doctor. Soon Daleks, Cyberman, Sontarians and lots of other Doctor Who aliens appear. They have formed an Alliance to defeat the Doctor because they know the Tardis will explode and destroy the universe.
The Pandorica opens. It is empty. They drag the Doctor towards his prison. He shouts that the Tardis will explode anyway and he is the only person who can stop it. They shut him inside anyway.
Auton Rory shoots Amy who appears to die.
The Tardis explodes with River inside it.
Around Earth all the galaxies disappear from existence.
So what do we know about the concluding part? the Radio Times interview with Karen Gillan implied she was filming the new series this summer so I assume she survives. Also a promotional image for the next episode shows the Doctor with 7-year-old Amy so perhaps somehow he goes back to stop things from happening.
Very good episode. In particular I felt the music was of movie-quality and made good use of the "Matt Smith theme" that was introduced in this series.
I look forward to the conclusion.