SPOILERS BEHIND THE CUTS
Through a complicated chain of events, a message for The Doctor is sent across time through a string of colleagues; from the visionary Vincent Van Gogh through a painting delivered to Winston Churchill to a phone-call rerouted to River Song to an ancient message carved into a mountainside at the beginning of time, The Doctor and Amy are sent to a Roman-Empire era Britain. It is there, under Stonehenge, that River has discovered The Pandorica - a legendary artifact spoken of in the legends of all sentient races that is said to contain the greatest threat to life in the universe.
Things get complicated when The Doctor discovers that The Pandorica is broadcasting a signal across time and throughout the universe and that a grab bag of his greatest enemies are en route to seize The Pandorica. Throw in the fact that Rory - thought dead and erased from time - is now living among the Roman soldiers stationed nearby and that River and the TARDIS become lost in time and space following a trip to 21st Century Britian and things go beyond complicated. And when an alliance unseen leaves The Doctor trapped, Amy dead and the entire universe minutes away from never having existed in the first place, things go so far beyond complicated that we need an entirely different word to describe just how complicated things have become!
THE GOOD PARTS
* The true nature of The Pandorica. While I guessed fairly early on that it contained The Doctor ("...Goblin. Or a trickster, or a warrior. A nameless, terrible thing, soaked in the blood of a billion galaxies.") I was not expecting it to be empty at the moment or that it was the result of every single Doctor Who villain teaming up to contain The Doctor after it becomes apparent that he is - somehow - responsible for the increasing destabilization of the universe. It's a concept that has been explored before - that The Doctor causes more trouble than he fixes - but it's never been presented in quite so staggering a manner.
* Matt Smith gets a lengthy speech worthy of The Doctor which manages to be all his own even as it gives literal shout-outs to battles fought by the the Ninth (I - AM - TALKING!) and Tenth (IT - IS - PROTECTED!) Doctors.
Come on! Look at me! No plan, no back-up, no weapons worth a damn! Oh, and something else I don't have: anything to lose! So! If you're sitting up there in your silly little spaceships with all your silly little guns and you've got any plans on taking the Pandorica tonight, just remember who's standing in your way! Remember every black day I ever stopped you and then, and then... do the smart thing: Let somebody else try first.
* Rory really comes into his own as a character in these episodes, proving to be all too human even as a robot. (Trust me - that makes perfect sense if you've seen this episode.) The Boy Who Waited Indeed.
* The only real problem with this story is that - by all rights - it would have been the perfect final Doctor Who adventure. The bittersweet near ending, in which The Doctor comforts the young, sleeping Amelia Pond as part of one last good deed before seemingly heading off to save the universe one last time by sacrificing himself to seal the hole in reality, leaving behind a more mundane world free of the threats of Dalkes and Cybermen... that would be a great way to end the series if it ever must end.
* The whole twisted logic chain in The Big Bang in which Steven Moffat indulges in the sort of time-travel trickery more often associated with Bill and Ted than Doctor Who. And unlike the last time Steven Moffat did this in Curse of the Fatal Death, it's not all that funny. That being said, I did like The Doctor running around in a fez.
The Final Verdict: A rare season finale that would be a serviceable series finale as well. It has very few problems, save that this story would be a perfect ending but it isn't an ending and that the comedic use of Time Travel really doesn't seem appropriate given the dire circumstances at the time. Still, these two episodes made the whole season worthwhile. Even Victory of the Daleks.