Moon rocks are all the rage and King Arthur is convinced he has traveled to the moon via a one-way rocket. While Sir Launcelot and Sir Martyn struggle to share the throne in the King's absence, Arthur, Merlin and Splag are captured by the fierce 'moon people' and do battle with the gigantic rock giant, Ritho, in order to bring Princess Guinevere a moon rock all the way from - North Wales.