This film is somewhat loosely based on Poul Anderson's novel of the same name. The premise of the story is that, just like in all those B-grade movies from the 50s, the Earth has been invaded by technologically-superior aliens bent on conquest – during the Crusades. Thus, the aliens happened upon a harmless little village whose menfolk all happened to be armed to the teeth and ready to set off to war. So the aliens' death rays are no use against shining armour, and their anti-death-ray defences are no use against an English longbow. The locals capture the ship, and the local baron decides that it would be useful in the crusade to liberate Jerusalem from the paynim. On the way to Jerusalem, they get a bit, um, lost.

There is some Christian content, in that one of the main characters is a monk, but mediaeval Christianity (or at least the producers' idea of it) is made fun of, although more gently than one might expect. But, basically, what mention of religion there is acts as "set dressing." What this film boils down to is a slapstick send-up of alien invasion flicks.