The backstory: a miniscule Viking tribe consisting entirely of people in their twenties sends all its menfolk off to sea in a random direction for some unexplained reason. Three years later, the women, plus one token man who didn't go on the earlier expedition for no reason that's ever explained, get a bit lonely and go looking for their men. It turns out there's a huge whirlpool patrolled by Nessie within easy rowing distance of their tiny Nordic homeland, and everyone sucked into it either dies, or is cast up exhausted on shore and enslaved by an evil Germanic furry-hatted tribe of whom there appear to about two dozen at most, and made to inefficiently chip rocks in a cardboard cave for no apparent reason.
The frontstory: people who can't act, including the most weary-looking villain you ever saw, whose every moment on screen exudes an air of "please pay me and let me get out of here!", furtle about trying to instill some sort of life into dismal dark-age clichés filmed on a budget of pocket change, and fail miserably. Jonathan Haze almost manages to do something interesting with his rôle as the wimpy son of the incredibly macho chief (I suspect the Monty Python team took notes, and used them in the development of Prince Herbert of Swamp Castle), but the rudimentary script won't let him. And the "Great Sea Serpent" is so briefly present and so cheaply animated that, until some token hastily-tying-up-the-loose-ends scenes at the end, I assumed it was stock footage from another movie.
This is Roger Corman at his laziest and most cynical. Never mind his cult reputation; this movie is obscure for a very good reason: it's absolutely abysmal! On the plus side, it runs for barely more than an hour. But it seems much longer. And not in a good way. Avoid.