To begin with, I expected this to be a modern political lampoon rather than yet another medieval (and typically bawdy) romp! With this in mind, the beginning is somewhat shaky but, eventually, it settles down to being a quite funny episode in the TV series.
Jack Douglas is assigned to procure a secret weapon devised by monk Kenneth Connor to aid in the Saxon king's war against the Normans. The journey back is fraught with peril even if the appointed assassin of the two travelers botches his job at every turn; a second spy, femme fatale Joan Sims, fares no better. David Lodge is featured as the Norman king.
The best gags here involve an old hag who seduces Douglas and Connor in the forest (of course, they reject her advances but, unbeknownst to them, she's actually acting on behalf of a virginal damsel!) and the climactic sequence, which has Connor's invention (a glass helmet, pretty much like an astronaut's!) ultimately prove fatal to the king so Douglas and Connor contrive to arrange an accident which would fulfill a prophecy attached to their sovereign's demise (and which also gives the episode its title)!