Several hundred years ago, humans were nearly exterminated by giants. Giants are typically several stories tall, seem to have no intelligence and who devour human beings.
A small percentage of humanity survied barricading themselves in a city protected by walls even taller than the biggest of giants.
Flash forward to the present and the city has not seen a giant in over 100 years - before teenager Elen and his foster sister Mikasa witness something horrific as the city walls are destroyed by a super-giant that appears from nowhere.