What Are The Best Big O Fanfiction Stories With Complex Mysteries?

I have a soft spot for stories that make the megadeuses part of the mystery, not just the solution. There's a WIP called 'The Bell Tolls for No One' that frames Beck's appearances as supernatural occurrences tied to a specific district's forgotten history. The mystery unfolds through environmental clues and architectural anomalies Roger notices, which is a clever way to build tension. It feels like a point-and-click adventure game in text form. The complexity comes from the dual investigation: Roger's on the ground while Norman digs through sealed corporate records, and their findings don't always match up, suggesting a larger cover-up. It hasn't updated in a while, which is a shame because the central conceit—that the city is a literal machine resetting itself—had so much potential.

Good luck. Most Big O fanfiction is either romantic fluff between Roger and Dorothy or crossover mecha battles. Complex mysteries are rare. I remember one called 'Grey Forty Years' that had an interesting premise—exploring the idea that the memory loss happened in waves, not all at once, and some people on the fringe of the city retained contradictory fragments. The mystery was about reconciling those fragments, and it used Angel pretty effectively as an ambiguous figure manipulating both sides. The plot got a bit convoluted near the end, but the atmosphere was spot-on, all rain-slicked streets and lonely late-night diner conversations. It’s probably still floating around on some older archives, not necessarily the big sites.

Honestly? I spend way too much time digging for this specific flavor of Big O fic. The anime's noir aesthetic practically begs for layered mysteries, but a lot of fics just transplant the characters into a generic detective plot. The real standouts are the ones that expand the show's own buried lore.

I kept coming back to 'The Memory of Sirens' by Veritas. It's an old one, probably on FFN and AO3. It builds a new case around a series of murders linked to forgotten megadeus models, with Roger and Dorothy uncovering fragments that point to something older than the 40-year memory gap. The mystery isn't just about whodunit; it's about how the city's amnesia functions as a systemic cover-up, which feels very true to the source. The author weaves in elements from the 'R.D.' faction smoothly, making the political conspiracy feel tangible.

Another is 'Automatic City' over on AO3. It treats Paradigm City itself as the central puzzle—why are there gears in the sky? The story introduces a new category of androids with partial, corrupted memories of the Event, and Roger has to piece together their testimonies while Norman provides dry, archival counterpoints. The pacing is deliberately slow, like the show, with revelations feeling earned rather than explosive. It leans hard into the psychological mystery of identity, which Dorothy's character arc explores perfectly.

Finding these is a chore, though. Tagging is inconsistent; you have to search for 'noir,' 'mystery,' and 'amnesia' alongside the pairing tags. I wish more writers would play with the show's core existential questions instead of just making Roger a simple P.I.

Try searching for 'case fic' as a tag on AO3, combined with 'The Big O'. That's how I found 'A Handful of Dust', which is a tight, single-case story involving a missing memory capsule. The mystery hinges on Dorothy's logical analysis contradicting the physical evidence, raising questions about perception. It's not an epic, but the puzzle is clever.