It's a post-Reichenbach fic which starts shortly after Sherlock's dive off the roof of Bart's. John is grieving, and wondering whether he could have had more with Sherlock than just friendship. There's grief, and angst, and wishing, and Sherlock-in-his-head until one day John decides that he can't go on like that; he needs to move past this and get on with his life. And that's exactly what he does.
John's stories about his days with Sherlock are very well-received in the newspapers, leading to a book deal. His publisher's marketing/ publicity agent, Mary Morstan, comes into John's life at this point and John starts believing he can build a life with her.
Then one day, out of the blue, John finds a coded message in the classifieds. He recognises the code from the case of the Blind Banker; it leads him to attend the arrest of a serial killer. Suddenly the game is on once more, and while John may not be a player this time he's certainly a very interested spectator while he tries to figure out who, exactly, is sending him these coded messages, and why.
I'm actually not giving away much here, even though it may seem I've just given you the entire plot. Much of the story is spent in John's head, detailing his thoughts, which adds to the richness of the story and really makes it what it is. Having said that, it's also my one criticism; in places, there is so much time spent with John's thoughts between him being asked a question and then him actually answering it, that I find I've forgotten what the question actually was and I have to scroll back to see. But that's a minor nitpick.
At 157357 words / 58 chapters it's the sort of fic you can dive into for hours at a time, and is well worth the time spent. Also, being on A03, it's very easy to download to kindle etc.
Ivyblossom's The Quiet Man here on AO3