Here's a short summary from Wikipedia regarding what to expect from the movie:
AD 117: The Roman Empire stretches from Egypt to Spain, and east as far as the Black Sea. But in Scotland, the relentless onslaught of conquest has ground to a halt in the face of the guerrilla tactics of an elusive enemy - the savage and terrifying tribes known as the Picts.
Centurion Quintus Dias, the sole survivor of a Pictish raid on a Roman frontier fort, is rescued by the legion of Titus Flavius Virilus, which has been ordered into Caledonia to wipe out the Picts and to kill their king, Gorlacon. When the legion is wiped out in a surprise attack, Dias and six other survivors race to rescue their captured general before they can head back toward the safety of the Roman frontier.
Centurion comes to theaters August 27, 2010 and stars Michael Fassbender, Dominic West, Olga Kurylenko, Noel Clarke, David Morrissey, JJ Feild, Axelle Carolyn, Riz Ahmed. The film is directed by Neil Marshall.
You can go back here to watch Centurion online for free on or after 27th of August.