Prince Modred and Aggravayne tell king Arthur that Sir Launcelot and Queen Guinevere are having an affair.
King Arthur banishes Sir Launcelot after supposedly kidnapping his Queen and killing his knights, however, Launcelot was actually saving the Queen from getting burned .
Siege of Benwick
King Arthur leaves his kingdom in the hands of Prince Modred and comes to France with Sir Gawain to battle with Sir Launcelot, he is doing this because Sir Gawain wants to avenge the deaths of his brothers.
During the same time, Prince Modred falsely declares King Arthur dead and takes his throne, he tries to marry Queen Guinevere (his stepmother) and she leaves to go to the tower of London. King Arthur finds out about this and decides to come back to England.
The Death of King Arthur
Prince Modred and King Arthur battle for the throne, King Arthur stabs Modred with a spear and Modred slashes King Arthur on his head, King Arthur dies later that day and is buried in a chapel that is run by the Archbishop of Canterbury.