The Doctor and Amy find out that River has committed a literally legendary act of graffiti that guides them to Salisbury Plain, Roman Britain. Amy immediately begins fucking literally every roman soldier she can see with the sole exception of Centurion Roranicus Pondicus. Funny they call him a centurion, when he seems to be more of a pedes or a tesserarius. An optio, maybe, at the very highest.
The Doctor and his bitches ride to Stonehenge because it's about time they dealt with it on the show. It turns out that underneath Stonehenge is a terrible sight: Amy's enormous magical box, which is desperately calling out to be filled by anyone and anything, but will only be satiated once it has the Doctor inside it. A cyberzombie attacks, and it's one of the only times in all 52 years of the program that a cyberman has actually been an interesting, intimidating monster. So, of course, the Cybermen have basically nothing to do with the rest of the plot.
The Daleks, the Cybermen, the Manlets, and a bunch of names grabbed off the Tardis Data Core (seriously, Chelonians? I'd have paid real money to see a deleted scene on the DVD where a chelonian was in the crowd shot) are all swarming around in the air overhead, so the Doctor is forced to give the most rousing speech he can muster, nearly spraining something in the process. In return, the assembled hordes of aliens graciously give them time to fill with character bits, plot, and River investigating the mystery of Amy's box.
It turns out the romans are Autons, Rory is actually a duplicate that ended up stealing the real Rory's soul, and Amy realizes the true identity of the nice guy who she's been ignoring all this time just before he murders her (ladies, take note). The aliens grab the Doctor and stuff him in the Pandorica as he's screaming about how he's a very important person everybody should be listening to. The River's silence programming kicks in and she blows up the Tardis. The ensuing explosion destroys the universe, blowing up every star throughout time and space simultaneously.