After his hometown is destroyed and his mother is killed, young Eren Jaeger vows to cleanse the earth of the giant humanoid Titans that have brought humanity to the brink of extinction.
Humans are nearly exterminated by giant creatures called Titans. Titans are typically several stories tall, seem to have no intelligence, devour human beings and, worst of all, seem to do it for the pleasure rather than as a food source. A small percentage of humanity survived by walling themselves in a land protected by extremely high walls, even taller than the biggest of titans. Flash forward to the present, and mankind has not seen a titan in over 100 years. One day, 10 year old Eren and his childhood friend Mikasa witness something horrific as the city walls are destroyed by a colossal titan that appears out of thin air. As the smaller titans flood the city, the two kids watch in horror as Eren's mother is eaten alive. Eren vows that he will murder every single titan and take revenge for all of mankind.
— Hajime IsayamaThe Battle of Trost District is the second arc of Attack on Titan. While it certainly packs a punch when it comes to intensity, it is lacking in the story department. This arc takes place when the students of the 104th Cadet Corps take on the Armored and the Colossal Titan in a fight.
The Clash of the Titans arc is, quite possibly, the arc with the biggest twists in the story. The arc begins when Wall Rose is suddenly breached and Titans appear out of nowhere. Immediately after, the Survey Corps are thrown into battle and it doesn't take them long to realize that the threat that they face is actually from within their own group.
he Royal Government Arc is slightly different when compared to the other arcs in Attack on Titan. This one involves a lot more politics than the vast majority of the arcs, yet, Isayama makes it so that it becomes incredibly enjoyable to go through. It is during this arc that Erwin Smith plans to overthrow the entire Government.
The Marley Arc starts the second half of the story where the entire world of Attack on Titan is finally revealed to the fans. Things finally go beyond just Paradis, which was earlier believed to be the entirety of the world. Across the sea, the Empire of Marley is explored in great depth. It is revealed that they hold the power of several Titans, however, they come to the realization that this power is now obsolete.
Shortly after Eren's attack on Liberio where he declares war on them, there is mischief within the people of Paradis itself. A new faction emerges which plans to install Eren as the leader of the new Eldian Empire. At the same time, the people of Marley now begin their mission to gain the power of the Founding Titan by infiltrating Paradis.