The surviving humans use an all-important tool to fight off Titans. As you’d expect, the Titans eventually find ways to break through and bam, you’ve got yourself an action series. The Titans wiped out the entirety of the human race, save for those who managed to find safety behind three huge walls.
Everything takes place around a hundred years after the arrival of huge creatures ranging from about 4 to 60 meters in height known as Titans. The Attack on Titan universe is far from the nicest place around.