What is Arthur doing at the opening of the story?

At the opening of the story, Arthur is looking for a sword for his brother, who left his behind. When Arthur found their lodgings locked, he panicked and rode to the churchyard to take that sword for his brother. He found it unprotected, and pulled it easily from the stone.

He brought the sword to Kay, who recognized it and showed Sir Ector. Arthur explained how he had taken it, but doubted his father's insistence on its import. To prove his point, Sir Ector had Arthur return the sword, and then he and Sir Kay demonstrated their inability to pull it out.

When Arthur again pulled it from the stone, his father and brother fell on their knees before him. In answer to the boy's confusion, Sir Ector explained that he was not Arthur's true father, but was rather a protector chosen by Merlin. Uther Pendragon was Arthur's father.