James was destined from the time of his birth to be a Knight of the Cross but was also born a Changeling, one of the most powerful Denarians discovered this and enacted a plan to try to take out potential sword bearers from play, the Knights of the Cross being their biggest threat.  The Archangel Uriel, discovered this plan and also knew that Titania had in her possession the legendary 4th nail of the Cross but she wouldn't part with it without some payment.  Uriel while discovering this also discovered that the Denarians were planning to capture Titania and trap her power causing a war between the Courts that would destroy the world.  So together he and Titania hatched a plan to set James up as a decoy Summer Knight, allowing the Denarians to think that their plan of taking James off the board as far as being a KotC was a success.  James would get all of the powers associated with being the Summer Knight temporarily while he did his duties to the Summer Court.  Upon the Denarians attempting to take Titania out, James would rescue her from her imprisonment and find the people who hatched such a heinous plan.  At the completion of this quest, Titania would release James from his geas and role as the decoy Summer Knight, and would relinquish the 4th nail to him so that a 4th sword could be forged and she would allow him to continue to make use of the magic of her Court.  He Chose to be human after the quest was completed so as to further remove himself from the influences of the Fae Courts (and because I needed to free up refresh to buy the True Faith abilities). 

The question this raises for me is what is wrong with Titania?  First she's captured, after being forewarned, by beings that are nowhere near her power level in the nevernever.  Then she relies on an outsider playing at being the Summer Knight to rescue her.  Finally she not only gives him a powerful artifact but also permanent access to the powers of the Summer Knight without any allegiance to her court.  Where was the actual Summer Knight during all of this?

The most interesting thing about the whole thing to me, assuming no parts of it are retconned, is that she purposely decreased the powers of her court.  By giving power away with nothing in return she's creating an imbalance between the courts.  It seems she's become as batty as Aurora was.