It begins with Tony in his suit, in an abandoned Hydra site (in Siberia, I think) dying. He was betrayed by Steve Rogers. He passes out, knowing that he is dying and because of the secrecy that he had agreed to before going to the meet-up at that location, knows that there is no way there will be a rescue. He returns to consciousness to find himself 4 years in the past, at the aftermath of the alien invasion during which he'd redirected the nuke. Although he is unsure about what is happening to him--is he hallucinating now? was the future that led to his death a dream or hallucination itself? If he really has returned to his past, is it permanent?--he decides that since what he remembers doing didn't end well he cannot bring himself to trust most of the Avengers, and chooses to act differently this time. I'd only read the first half dozen or so chapters, but he seems to be headed to strengthening ties with Thor and Loki.
This is not one of my usual fandoms to read in, so I'm not sure I was picking up all the important changes about exactly what he was doing differently, as I know only the broad strokes of the canon plot.
The story is completed and I remember that although the author's notes at the beginning said that it would have about 20 chapters, it ended up having an even 100 chapters. I believe the completed length was between 200 and 300 thousand words.
If anyone recognizes this, I'd really like to be able to read the rest of it.