This is an absolutely excellent historical fiction film, granted with extreme license. OK, I am a Roman history pedant and aficionado of historical novels, so I brought to this a critical eye, but also a hope that I would get enveloped in the narrative, in subtle character, in wonder at the endless varieties of human ambition. At this, it was so successful that I forgot the present with complete delight, even after 2 full viewings.
In terms of history, this is fiction based on speculation about what happened to the 9th Legion, which disappeared from the historical record without a trace - the only way to limn what happened to them is to piece together ambiguous clues, such as the inscriptions on grave stones scattered throughout the former empire. The time is that of the Emperor Hadrian, the golden age of the Roman Empire - the series of good, circumspect emperors who governed judiciously and without the insane excess that is remembered with Nero or Commodus. Rome is one of the largest stable empires the world has ever known, though it has experienced resistance in certain areas, such as the Picts in Northern Britain or the Parthians in Central Asia. Whatever happened, the film that concocts an explanation, and it is as plausible as an other, with plenty of fictional speculation and fantasy thrown in to build a narrative.
To be honest, the film either works for you or it doesn't. All I can say is, if you liked HBO's Rome, this is the same kind of grainy, highly textured portrait, just not in the center (Rome) but on the periphery. I believe this preference is personal and subjective, a pure matter of taste. It is extremely bloody, with realistic battle sequences that are not for the squeamish.
The plot follows a non-commissioned officer (Centurion), acted by the wonderful Michael Fassbender. Long a fighter of Picts (he knows their language), he survives an attack on an outpost and is rescued by the 9th, who are about to make war on the Picts. They are led by the incredible Dominic West (of The Wire), a charismatic general who has led his men to glory in Spain and is now establishing them in Britain; he is a gifted brute, but extremely popular with the men, from whom he rose by talent. Their scout is Olga Kurilenko (yes, a Bond girl), a mute and mysterious young Pict.
After the quick decimation of the 9th, Fassbender and a handful of survivors seek to rescue the general by duty, deep in Pict Scotland. Upon failure and an unfortunate murder, they must flee for their lives while an elite corps of Pict warriors pursue them with a blood oath of vengeance. At their head is a killer of singular talent, whose family was brutalized and executed before her eyes, then she was raped and left for dead. As described, "she is an empty vessel that can only be filled with Roman blood." When you see her, you believe it. I will not reveal what happens, only to say that it is acted to utter perfection.
There has been much criticism of the Romans as "good guys" in this film. I think it is far more subtle than that: no one is good, no one bad, they are all just striving to live the way they want, in accordance with their traditions. The Picts are genuinely savage warriors, but the reasons for their ruthless ferocity are completely clear: the Romans brutilized them in unspeakable ways as demonstrated more than once. For their part, the Romans are simple soldiers, part of a military machine the likes of which the world had never known. They are not good guys, they just are what they are: disciplined killers who will follow their leaders to the death for duty and hopefully glory. They are tougher beyond imagination, with their own codes of honor, though clearly one of the survivors is a sociopath. Another much criticized plot incident is the girl and love story. I suppose you could say it is unrealistic, but what the heck, it is a movie. To me, it was believable and moving (in however a superficial way you might want). Roman political machinations are also surprising, believable, and subtle.
This is not the kind of thing a pedant like me should pick apart. It is just a darn good story that completely absorbed me for 2 viewings. I will certainly watch it again. Warmly recommended.