The Fire Nation in 12 BG is a prosperous, industrializing island empire. Sozin has been Fire Lord since 58 BG — forty-six years. He is seventy, the same age Roku was. In public he is a reformer who brought noble clans to heel early in his reign, put poor citizens to work on a coal fleet, and now mourns the nation's Avatar. In private he has just left his oldest friend to die, because the last leash on an empire is gone.
THE COURT
Hari Bulkan, the Capital in the Caldera, is the throat of the nation. The old throne room was destroyed by Roku in 37 BG and has been rebuilt. Sozin works most days in the Private Council Chamber, not in public audience. Fire Lady Kaida (original name; Sozin's wife is unnamed in canon) holds the Consort's Garden Pavilion and is better at statecraft than she is allowed to appear; she already suspects the official story. Lord Minister Hesun, an original official, runs the Ministry of Imperial Planning's windowless archive and, through it, the Crimson Mandate. General Rantai believes the next war will be defensive and has already received contingency invasion tables for western Earth Kingdom roads. Grand Admiral Keshiro is proud of the Black Sun and the drydocks; shortages are being blamed on Earth merchants by design. Sage Ruga, in the Royal Observatory's comet-chart loft, has already dated the next sky-fire to twelve years; Hesun has classified the charts.
THE OFFICIAL STORY AND THE PEOPLE WHO CAN BREAK IT
The palace says Roku died a hero and Sozin could not save him. Captain Viro at the Main Caldera Gate logged a dragon returning without the Avatar's dragon and was told to lose the line; a copy is in his mattress. Private Sen saw him hide it. Kenji, in the servants' wash-yard, smelled a cloak of sulfur and not of a long fight. Dockhand Reki marked a crate of 'mourning incense' that clinked like iron. Nara Jin saw manacles under rice on Azane's 'relief' cutter. Ferryman Batsu may have carried a hooded woman with a wrapped headpiece. Madam Sora has already hidden two island witnesses in The Ash Lantern attic. Lieutenant Azane is collecting those witnesses. Ta Min, on the Refugee Quay and in the Old Caldera Townhouse, cannot shake the timing of Sozin's departure and will not yet accuse the Fire Lord aloud. Rina, their daughter, is packing in the cellar shrine and preparing to disappear (canon later hiding). The Crown Prince headpiece — Sozin's old gift to Roku — is with her. That object is political dynamite.
SAGES
High Sage Onaru wants to preserve the order and has not signed Hesun's draft emergency allegiance, yet. Fire Sage Kaja, very old, keeps the Crescent Island sanctuary for the next Avatar and will turn away soldiers who will not bow to Roku's memory. The sages' later fall into the Fire Lord's service is not complete. This decade is when it can be forced, resisted, or delayed.
DRAGONS AND THE ARTS
Sozin's unnamed blue dragon stalls in the Royal Dragon Aerie, uneasy. Dragon hunts continue in the countryside. Academy fire is sharp and martial. The Dancing Dragon survives among the hidden and the stubborn. Sozin himself is a prodigious bender and, in canon, uniquely known for redirecting heat.
FOREIGN FOOTPRINT
Envoy Sana keeps the Water Tribe house in the Foreign Envoy Quarter and already does not believe the official island story. The Fire & Air Center of Learning, once a celebration of Air teachings in the Fire Nation, has a militarized history with the Guiding Wind; Sister Palden still tries to keep a courtyard of shared learning open. Princess Zeisan, Sozin's nonbender sister, lives in austerity at Ashfern Hermitage with Sister Rioshon, still trying to blunt her brother's worst ambitions.
Play the Fire Nation as a place many decent people love, already being aimed like a weapon.