A young, unknown boy named Arthur becomes King of England, and his noble parentage as the son of King Uther is revealed. He defeats an alliance of twelve northern kings who contest his succession.
The big challenge for Arthur at this point is simply to establish his (double!) legitimacy as king. He accomplishes this when Merlin verifies his parentage and he defeats the alliance of kings who are still protesting his succession. But just proving legitimacy is not all it takes to be a great king – more is left to be accomplished, and that's where Arthur turns his attentions next.
With his marriage to Gwenyvere, who brings with her a round table and whole boatload of knights, Arthur enters the beginning of his golden period. He has 150 knights who not only swear allegiance to him, but also swear an oath to uphold certain rules of chivalry. These knights travel all over the kingdom winning renown for and bringing glory to Arthur.
Arthur ignores the love between Launcelot and Gwenyvere, and he fails to protect Lamerok from murder at the hands of Lot's sons. Fractures appear among his knights. Arthur's unwillingness to confront Launcelot and Gwenyvere over their love, despite obvious evidence, will eventually be responsible for the total downfall of his rule, as will his failure to put an end to the feud between the families of Lot and Pellynore. If only we could drop him a hint!
Aggravayne and Mordred expose Launcelot and Gwenyvere, forcing Arthur to bring Gwenyvere to justice. In the ensuing battle, the factions in Arthur's court face off. Gawain convinces Arthur to go to war with Launcelot, and while they are in France, Mordred takes control of Arthur's kingdom.
Ugh, it's all just a big mess now isn't it? Aggravayne and Mordred's decision to expose Launcelot and Gwenyvere moves all decision-making power out of Arthur's hands; if he fails to punish her and Launcelot, he'll look like a weak king. Arthur's powerlessness continues when, for some reason, he can't seem to say no to Gawain's demand that he go to war with Launcelot. This opens up a power vacuum that enables Mordred to seize the throne. Things are not looking good for our guy.
In the end, Mordred gives Arthur his death-wound on Salisbury Plain. Arthur's death is caused by forces he has set in motion in more ways than one: (1) His war against Launcelot, which allows Mordred to seize the throne, was partially a result of his failure to control his own knights (and his wife, for that matter) and (2) Mordred, his killer, is the product of his incestuous relationship with his sister long ago.