Sigurd Pål (Sverre Hagen) is a Norwegian archeologist, raising 2 children alone.  He has found some ancient runes that he thinks lead to a treasure.  When he presents this to the Museum board, they ridicule the idea.  They cut his funding and demote him.  He convinces his colleagues Allen (Nicolai Cleve Broch) and Elisabeth (Sofia Helin).  The three archeologists head off to search for the treasure in a remote area in far northern Norway, with Sigurd's 2 kids along.  They hire a local man, Lief, to take them to an island.  There is a cave on the island, where the runes indicated.  They find Viking helmets, swords etc...

Sigurd's son finds a huge egg in the cave and brings it with him. Leif takes the treasures and abandons the others in the cave.  As he crosses the lake alone, he is killed by something that comes up from the deep.   That night, the egg hatches and a serpent like creature emerges.  They trap it in a metal toolbox.  They are attacked by a huge serpent.  Sigurd realizes the runes were not a map to treasure, but a warning of a dangerous area to avoid.

Allen, like Lief, abandons the others in a cavern to return alone.  He is killed by the serpent.  The serpent traps the 2 kids in a metal locker.  Sigurd returns with the baby serpent in the toolbox.  So, each dad has the other one's young trapped in a metal box.  Sigurd gives the baby serpent back to its parent.  The big serpent leaves.  Sigurd, Elisabeth and the 2 kids survive. 

Thanks Joseph C!