Several hundred years ago, humans were nearly exterminated by 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 city protected by extremely high walls, even taller than the biggest Titans. Flash forward to the present and the city has not seen a Titan in over 100 years. Until that day.....

Here are some helpful links if you want to read the manga.

Live Action Movie 2015-08