I can repost a summary someone asked for earlier.
Close to 2,000 years ago, a woman named Ymir came into contact with the origin of organic life and became a Titan shifter, also bringing regular Titans we know into the world. On her death, her power split into 9 different unique titans; whoever has this power dies within 13 years, like Ymir did, though it can instead be passed on by a mindless titan drinking the spinal fluid of the shifter. Ymir and her people are Eldians, who can be turned into titans with some cheap drug. They conquered the land in the past and were ruled by the Founding Titan who became the royal Fritz family. The Founding Titan both passes along the memories of its past holders while also allowing the user to manipulate memories of people in its bloodline. It can also command all other Titans.
The 145th Fritz King decided to move the nation to a small island and construct walls to protect his people. He commanded large Titans to harden themselves into walls, and altered all his subjects memories to believe that all life outside of the walls had died to the mindless Titans. The few nobles he didn't control kept themselves quiet.
Outside of the wall, the nation of Marley took control of old Eldia and of the other shifter's abilities that the King left behind, constructing ghettos for the remaining Eldians to live in. Think 20th Century Europe with Jews, checkpoints, armbands. People who break the rules get imprisoned, boated over to the Eldian island, and injected with the drug to turn into Titans let loose outside of the Walls.
There's a movement among some remaining Eldians in Marley to rise to power, but it fails when Eren's father, Grisha, is betrayed by his son Zeke. Grisha gets carted to the island. His wife gets turned into the smiling Titan who eats Eren's mom, but Grisha is saved by the Attack Titan who has Grisha gain his power and go inside of the walls to convince the king to rejoin the rest of the world.