Synopsis:
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 of titans. Flash forward to the present and the city has not seen a titan in over 100 years. Teenage boy Eren Jaegar (Yûki Kaji) and his foster sister Mikasa Ackerman (Yui Ishikawa) witness something horrific as the city walls are destroyed by a colossal titan that appears out of thin air.
http://www.funimation.com/attack-on-titan/videos
Watch the first episode. It takes 24 fucking minutes. If you are not hooked, then I'll let you insult me.
Also, here's the intro/opening to get you hyped:
http://www.youtube.com/watch?v=8jTHfdgCiDU