Sometime around when Avatar: The Last Airbender takes place, Zuko and Katara have sex. Katara is impregnated. At some point, he gives Zuko her necklace to show her love for him.
Zuko takes up duties as Fire Lord and marries Mai.
During the period between this and the beginning of the comic proper, the following things happen (order unknown):
Ozai is executed, and Azula has her bending removed and is banished.
Aang finds a temple of Air Nomads who survived the war.
Toph inherits her family's estates. She, Katara, and Sokka live in one.
The Northern and Southern Water Tribes merge. Katara and Sokka become prince and princess of the new nation.
Sokka tries to send a messenger hawk to the Northern Water Tribe with this information, but accidentally sends one to the Fire Lord, despite Katara not wanting Zuko to find out about her pregnancy. Zuko, however, doesn't find the letter- his wife Mai does. Mai keeps the letter from him and acts to remove the competition by poisoning Katara and aborting her child.
After the above event, Katara gets a basket of fruit, supposedly from Zuko. It's actually from Mai, and when Katara takes a bite from what appears to be an orange, she immediately suffers a miscarriage.
Azula attempts to blackmail Mai.
Some years later on the anniversary of this miscarriage, Katara and Toph go to Kuzon's memorial. Katara angsts about losing Kuzon and not being able to confess her love to Zuko. Meanwhile, in the Fire Nation, Zuko also angsts about his love for Katara.
In the fire nation, the maid Lin discovers the letter that Sokka sent and Mai kept.
While Zuko is writing a letter and angsting about his star-crossed love, Iroh interrupts, introducing Lin and telling Zuko about the letter she found. Iroh tells Zuko that none of his letters to Katara have been getting through, and that Katara was pregnant with his child.
Zuko confronts Mai over the letter. They argue, and in the end, Zuko declares Mai no longer his queen.
Azula's blackmail attempt goes horribly wrong, and she is left poisoned and with a nasty wound to her side.
The night before her birthday, Katara has a dream about her miscarriage.
Sokka finds Azula, injured and wearing a ninja costume, on the streets. When she falls unconscious, he picks her up and takes her to Katara for healing.
Aang talks to the air nomad Ping about his love for Toph.
Katara's birthday ball is held. Katara meets with several minor canon characters. Aang and Toph meet and confess their love for each other. They then kiss. Later, Zuko comes, and when he notices Katara, they run towards each other and kiss.
Immediately following this, Katara curses him out for leaving her and never returning her letters whilst plagiarizing Rascal Flatts. Zuko explains that Mai was the reason behind both her miscarriage and her lost letters, and the two kiss again.
After the party, Sokka comes across a woman in scraps of a ninja costume, coated in bruises and severely poisoned. After healing her, Katara tells one of her maids to watch over her and make sure she's alright when she awakens.