Unmarked Les Misérables spoilers below! You have been warned.
In short, the premise is that the timeline diverges from the book when Valjean rescues Javert from the Seine and nurses him back to health, then they eventually fall in love. This is a fairly common Fandom-Specific Plot for Les Mis, known as "Post-Seine", but AROS is by far the longest fic of it (at least of those hosted on AO3).
A Reflection of Starlight provides examples of:
- Arc Words: Variations on the exchange "You are a fool" "There are worse things to be" between Javert and Valjean have occurred four times so far: in chapter 2, chapter 36, chapter 64, and chapter 66. Should probably expand on their contexts
- Door Stopper: As of August 2018, it's 66 chapters long, with a total of nearly 375 thousand words, and it's still ongoing. For scale, the Brick is 655 thousand words.
- Fix Fic: This fic fixes just about everything wrong with the canon denouement.
- Happily Failed Suicide: Javert is initally angry at Valjean for saving him, but this eventually applies.
- Interrupted Suicide: This fic's initial canon divergence
- Laser-Guided Karma: In canon, Thénardier gets away with everything. Not so here.
- Unresolved Sexual Tension: Well, not sexual as such (the author headcanons them both as some form of asexual), but the romantic tension between Valjean and Javert remains unresolved for a VERY long time.