Warning: The following contains spoilers for Attack on Titan by Hajime Isayama, available now through Funimation.
Attack on Titan has gone down a long and winding road since its first season. The struggle of mankind to survive the looming threat of the man-eating giants has taken on all sorts of shapes and forms. The fighting went from human-vs-Titan to Titan-vs-Titan to human-vs-human and just keeps alternating between them all. There are even those who can shift between Titan and human form to blur the lines even further. Now, with the Final Season, everything is coming to a head and it's chaos all around. To help better understand the conflict, here's an overview of everything that's happened in Attack on Titan thus far.
The initial premise of Attack on Titan was a fairly simple one. The apparent last of humanity had sequestered itself behind enormous Walls to avoid being brutally devoured by Titans. Eren Yeager and his comrades in arms sought to fight the Titans and travel beyond the walls. In achieving the latter, however, they realized that the rest of the world is alive and wanted everyone in the Walls dead. With this, the pieces were all in place for the endgame, and the final act could begin.
There are three major factions ready to fight in the Final Season. The first side introduced is Marley. Using the power of the Titans, they had become the world's primary military superpower. However, technology had been steadily catching up to the Titans and threatened to render them obsolete. The Marleyans' goal was to take the absolute power of the Founding Titan from the Eldians who lived behind the Walls of Paradis Island and regain their edge long enough to advance their own technology.
Among the Marleyan military was a notable unit of Eldians desperate to prove their patriotism and atone for the sins of their ancestors. Notable members of this unit included Reiner (inheritor of the Armored Titan) Gabi Braun, Falco, Colt Grice, Porco Galliard (Jaw Titan), and Pieck Finger (Cart Titan). Gabi is introduced wanting to slaughter all Eldian devils but eventually learned to understand them better after living among them. Falco wanted to do whatever it takes to protect Gabi. Reiner is ashamed by the life he's led, but he lived on to insure that Falco and his friends have a better future than him. Annie Leonhart (Female Titan) was technically among these soldiers, though she remained crystalized and under the careful watch of the Paradis Eldians. There was also their commanding officer Theo Magath; he wasn't an Eldian, but he at least partially sympathized with them.
The second major faction was the Eldians of Paradis Island. They composed the main cast of the first three seasons, so there's not much new information to give about them individually. Their only deterrent against invading forces was the threat of The Rumbling, which was the use of the Wall Titans to trample the rest of the world. Their goal, however, was to find a nonviolent solution to prevent the rest of the world from going to war with them in the first place.
Eren Yeager (who has the powers of the Attack, Founder, and Warhammer Titans) had his own faction entirely. He had concluded that there was no way the conflict between Paradis and everyone else wouldn't end with violence. Furthermore, he believed the only way to deter enemy nations like Marley was through a show of force. His current goal involved meeting up with Zeke Yeager (Beast Titan) to use his royal blood to access the full range of the Founding Titan's capabilities, which included the use of The Rumbling. Eren's contemptible means of attaining freedom for Paradis and his friends, especially Mikasa and Armin (Colossal Titan), have changed him into the central antagonist of Attack on Titan.
Aiding in Eren's cause are the Yeagerists, or the New Eldian Empire. They, like Eren, were anti-military deserters who viewed their military's approach to handling enemy nations as too passive and sought immediate action and reformation. They were mostly made up of former soldiers like Floch Forster. Also on Eren's side were Marleyan defectors like Zeke and Yelena. Zeke, while working with Eren for now, was plotting to use the Founding Titan's power to sterilize the Subjects of Ymir and bring an end to the age of Titans once and for all.
With everything that happens by Episode 75, "Above and Below," the final battle begins where it all started-- in Shiganshina. It will be a three-way confrontation between Marley, Eldia, and the Yeagerists. It's also worth noting that eight of the Nine Titans are now in one place. The fate of the entire world hinges on the outcome of this battle. Eldia will either end up destroyed or liberated. The epic conclusion to Attack on Titan begins on Jan. 9, 2021.