"You're going to have to give up your first born son if you want to marry Lady Igraine."
"I can't find it! I'll go look for another one to replace it..."
"I wonder why this is here..." "Probably for people who need it can borrow it, i'll put it back later"
Uther (the king) fell in love with Lady Igraine, the only way that he could marry her was to give his first born son to Merlin.
Arthur lost Kay’s sword that he needed for battle so he went to look for another one and found “The Sword in the Stone”.
Arthur pulled out the sword without realizing that he just confirmed that he was the son of Uther and gave it to Kay.
Kay new that it was the magical sword in the stone and showed it to his father and tried taking the credit, he found out that Kay was lying but found out that Arthur pulled it out and would be the new king.
"You're the son of Uther!"
People start to realize that Arthur was the true son of Uther and would be their new king after multiple times of pulling the sword out of the stone against other candidates.
Arthur had the coronation to be king but the book left it at that point and ended.