Tony wakes up one night with a god of mischief in his bed and things go downhill from there. The Avengers end up caught in the middle of a fight between Loki and the Enchantress and at various points along the way have to decide who to trust in order to save members of their team. Tony especially must figure out if what's started between him and Loki is real or just part of the game.
This started before Avengers came out so a lot of core set up stuff has been Jossed and it's pretty much AU now even if it wasn't slash. It still captures MCU Tony so well I can hear his lines in RDJ's voice. The depiction of Nick Fury probably owes more to his sincerely nasty Ultimates incarnation than what we see of him in Avengers so be warned for that.
Great art based on the story: