So Marcus and Esca cross the Wall to hunt for The Eagle. Esca politely asks anyone they meet if they saw 5000 Roman soldiers walking by 20 years ago, and they keep walking north.
They get attacked a few times, but defend themselves quite well.
A little later, Marcus thinks Esca is not asking the right questions, so threatens a villager and finds out about a man who can help them. They soon find the man - a deserter - a legionnaire who ran when the fighting was fiercest and has been living in Caledonia, now married with kids. He points them on the way to the far north, but won't go with them.
On their quest they follow an eagle, to give them hope, and then bump into the ferocious Seal People when they finally grow sick of each other's company and fight. Esca takes control and tells the Chief's son that Marcus is his Roman slave. They are taken to the Seal village on the northern tip of Scotland, and Esca (as the Brigante Chief's son) is accepted.
The young Seal warriors prepare for a rite of passage and the men get drunk on moonshine. The Chief dons fancy dress and brings The Eagle out from a cave. In a trance, Marcus walks straight to it, and is soon knocked unconscious.
With all the men heavily asleep and worse for wear, Esca shakes Marcus awake, and together they enter the cave to get the Eagle. Marcus is pleased to have his old friend back. Inside they are startled by the Chief and his bodyguards, but they prove no match for Marcus and Esca. Before he dies, the Chief tells Marcus that he saw the Roman Commander-in-Chief beg for mercy like a coward before he killed him. Esca doesn't translate what was said.
They pack their horses, but the Chief's youngest son catches them. He wants to go with. Esca convinces him to stay behind and keep quiet, and the two are off.
In the morning the Chief's eldest son sees the gift Esca left behind with the young boy, and soon the cat is out the bag. A war party is summoned, and they race after the Roman and his slave.
The Seal People are tough, and with better knowledge of the terrain are gaining on Marcus and Esca. Soon their horses give in, and Macus's leg is giving him gip. Marcus stays behind near a submerged stream, hidden by rocks, with the Eagle while Esca goes for help.
Esca returns with the legionnaires who deserted the army 20 years ago, led by the wild man before. Just in time, as the Seal People have found their location. The Chief's Eldest kills the young boy in front of the Romans. Wrong move as this makes little Esca mad.
There is a final battle. The Romans are doing well, but the Chief's Eldest is a formidable fighting machine and dispatches the old legionnaires with ease. In fact if he took them all on alone he would be finished with his enemy in no time. He defeats the Wild Man. But soon he meets Marcus, and the experienced Roman gets the better of his enemy. Without their captain, the remaining members of the Seal war party give up and go home. Somewhere in all this, Marcus learns that his father died a hero - the deserters stayed with the ranks until the very end, but were ashamed to admit that before.
Marcus, Esca and the remaining few soldiers burn the body of the Wild Man on a funeral pyre.
Marcus and Esca say their goodbyes, return to Roman Britain and present the Eagle back to the Senator. Esca is now a free man and Marcus knows that his family name has been vindicated.
While burning the body of the Legionnaire, Marcus also sticks the Eagle onto the pyre in homage to all those, on both sides, that lost their lives.
The movie ends with the two friends walking back towards the Wall having a game of anti-quitsies, no erasies, triple-stamped tag Dumb and Dumber style. Don't believe me?