A small artifical cavern hollowed out of the chalk in the centre of the medieval market town of Royston (Herts, UK) is widely accepted as a shrine used by the Knights Templar, possibly continuing on in secret after the order was suppressed early in the fourteenth century. Meanwhile, conspiracy theories concerning Freemasonry, the supposed bloodline descended from Jesus of Nazareth and Mary Magdalene and other fashionable nonsense are linked with the cave. What are the facts about the cave and what can we reasonably deduce about its origins?