Uther Pendragon and Duchess Igraine had a baby but the duchess was not his wife so they needed to hide him. Conveniently Merlin a wizard took Arthur and brought him to Sir Ector to care for him.
Not to worry my king, I Merlin will take your child to Sir Ector and he will be raised under his care for the time being.
What are we gonna do about the baby?
Sir Ector and his son Kay took Arthur under their wings and taught him the way of a knight and cared for him like family. One day Sir Kay had a tournament and it was Arthur's job to bring the sword but he forgot it.
King Arthur became king and formed the Knights of the Round Table to protect the citizens.
How could you forget my sword? Especially on my big day! You better figure something out!
I'm sorry Kay it was an accident. I'll try to find another one. I'll be back.
I'm disappointed in you Arthur. After everything we've taught you.
After his mistake young Arthur travels into the woods in search of a sword. He finds one embedded in stone and pulls it out not knowing that he is now the rightful king and then goes back to find Kay.
How convenient! I'll give this sword to Kay.
All hail King Arthur!
Young Arthur was very confused and tried putting the sword back but even then no one could pull it out. He was the new and rightful King.