The newly crowned King Arthur is unsure of himself; worse, the people are unsure of him. Too many people want the throne, and treachery is everywhere. Merlin must do something before the king is betrayed, or murdered, or--worst of all--gets married. So Merlin magically places a sword into a slab of rock, lets it be known that whosoever removes the blade will rule all of England, and invites any man who would dare, to try to pull out the sword.
After a bit of showmanship, Arthur will draw the blade (with a little magical help, of course), and the people will rally around the young king.
Except someone else pulls the sword out first. . . .