I've had this bookmarked for ages, but I've only now just found the time to read it, and wow. It's stunning. Snarry is far from my favorite ship -- I only read it because there's so much great fic written for it! -- but this is the kind of story that brings the pairing to a sort of fierce, convincing life. I can't not believe in the rightness of it.
Lily is wonderful here, beautifully flawed, as are Harry and the other supporting characters (Dumbledore and Grindelwald in the afterlife make me positively gleeful), but your Snape grabbed me by the shorthairs and wouldn't let me go. He is perfect (meaning, of course that he's anything but).