McKay gains telekinesis from enzyme overdose.

I can't quite remember how it starts, but it's set during The Hive - after a massive overdose of the wraith enzyme, McKay develops psychic powers and boards the hive ship, does battle with the wraith queen and kills it, then has sex with Sheppard on her throne.
The story continues after that with a mission to some random planet where McKay saves some kids from a wraith attack with his psychic abilities. Oh, and I remember that using his abilities turns his eyes completely black, so he wears sunglasses to hide them.