In London in the year 1907, a British aristocrat, industrialist and millionaire named Sir Anthony Ross (Donald Sinden) hastily arranges an expedition to the Arctic to search for his lost son Donald. Donald had become lost on a whaling expedition to find the fabled island where whales go to die.
Sir Anthony employs the talents of a Scandinavian-American archaeologist Professor John Ivarsson (David Hartman) and Captain Brieux (Jacques Marin), a French inventor/aeronaut who pilots the expedition in a French dirigible named the Hyperion, which Captain Brieux invented. Upon reaching the Arctic, they meet Oomiak (Mako Iwamatsu), a comically cowardly/brave Eskimo friend of Donald’s, and trick him into helping them join in the search.
Ultimately, the expedition becomes (temporarily) separated from Captain Brieux, and discovers an uncharted island named Astragard, occupied by a lost civilization of Vikings, cut off from the rest of the world for centuries. The Vikings capture Sir Anthony and Ivarsson, but Oomiak escapes. Shortly thereafter they find Donald, but are nearly put to death by the fanatical Godi (pronounced “Go-dah”)[needs IPA], a religious soothsayer/authority figure.
The three men (Sir Anthony, Ivarsson and Donald) are saved from being burned alive by a brave and beautiful Viking girl named Freyja, with whom Donald is deeply and mutually in love. They escape, and are rejoined by Oomiak and eventually find the Whale’s Graveyard, but are attacked by killer whales. Here they are saved by the sudden reappearance of Captain Brieux, but they are still being pursued by the angry Godi and his rather unwilling warriors.
Finally, Godi is killed by the explosion when he shoots a fiery arrow at the Hyperion, but the Vikings will not let the expedition return to their world unless one of them remains behind as a hostage. Ivarsson however, willingly volunteers to stay, because this is a chance to live history. Ivarsson also points out that if someday mankind is ever foolish enough to destroy itself, places like Astragard may become humanity’s final refuge.
Sir Anthony, Donald, Freyja, Captain Brieux and Oomiak, are allowed to depart in peace, promising not to tell the outside world about Astragard. As Ivarsson heads back to Astragard, he turns to look back just in time to see his four friends move further and further away until they vanish into the Arctic mist.