A spacecraft crash lands in a large lake in Norway, circa 709 AD. From the wreckage emerges Kainan, a soldier, who was transporting (unknowingly aboard the ship) an alien monster, a Moorwen. After Kainan buries his captain he recovers a portable computer from the wreckage and uses it to find that he is on Earth (classified by his people as an "Abandoned Seed Colony") and there are no other interstellar vessels detected nearby. Determined to track down the alien, Kainan has the local language and knowledge painfully downloaded directly into his mind. He also sets a distress beacon to signal a rescue ship.
He begins tracking the Moorwen and discovers it has destroyed a village and killed its inhabitants; the village was in the territory of Gunnar, neighbor to the tribe who will take Kainan in. Kainan is captured by Wulfric, a Norse warrior who dubs him an "outlander", losing his weapon along the way. He is held prisoner in the burg ruled by King Hrothgar. Gradually, as the Norsemen believe his story about "hunting a dragon", and after he saves the King's life, he is accepted into their society. He also forms a bond with a silent orphan, Eric, and the King's daughter Freya.
In flashbacks, Kainan's backstory is revealed, involving an ongoing war in which man and alien both seek revenge for violence committed against them. Kainan's people came to the Moorwens' world to establish a colony, killing the natives and claiming the planet for themselves. As a reward, Kainan's wife and son were given a home in the new colony, while Kainan and the armed forces left for other conquests. Later, one surviving Moorwen slaughters all the colonists, including Kainan's family. Kainan was dispatched to capture the beast and was returning with it to his homeworld when it broke free and caused the crash.
As the Moorwen "dragon" ravages the countryside, Kainan forms an alliance with the warriors. The Moorwen's devastation triggers war between King Gunnar and King Hrothgar. Gunnar assaults Hrothgar's village, but his forces are repulsed. As his men retreat into the forest they are attacked by the Moorwen, forcing them to seek refuge with Hrothgar. Together, the two rival warbands attempt to destroy the Moorwen and its newly born offspring. Many of the Norsemen are killed, including both kings. Most of the Norsemen, now under Wulfric's leadership, leave the village in search of a new home and safety.
With metal salvaged from his crashed spacecraft, Kainan forges new weapons capable of harming the Moorwen. After a harrowing journey, Kainan and the warriors confront the Moorwen in a cave system and defeat the alien monsters. Only Kainan and Freya survive, with Wulfric as he dies transferring leadership to Kainan. Leaving Freya to rejoin her people, Kainan heads back to the lake where he crashed.
Kainan approaches his computer, which is still broadcasting the distress signal. Kainan spots a rescue ship entering the atmosphere, but decides to destroy the beacon with his sword, choosing to remain on Earth. The ship reverses course and returns to space. Kainan is unaware Freya witnessed the entire event. An epilogue explains that Kainan and Freya married and adopted Eric, and Kainan became the new king. Only Freya knew Kainan's "secret" - that the Gods had sent him to the Norsemen "and when the time came for him to return to them, he chose to stay with us instead."
Jim Caviezel as Kainan, a space traveller
Sophia Myles as Freya, daughter of Hrothgar
Jack Huston as Wulfric, nephew of Hrothgar and heir to the title of King of Heorot
John Hurt as Hrothgar, King of Heorot
Cliff Saunders as Boromir, Heorot's smith
Ron Perlman as Gunnar, king of the neighbouring village
Aidan Devine as Einar
Bailey Maughan as Eric, a young orphan