A few hundred years prior, people were about killed by titans. Titans are normally a few stories tall, appear to have no knowledge, eat up individuals and, to top it all off, appear to do it for the joy instead of as a sustenance source. A little level of humankind made due by walling themselves in a city ensured by incredibly high dividers, significantly taller than the greatest of titans.
Glimmer forward to the present and the city has not seen a titan in more than 100 years. Adolescent kid Eren Jaegar (Yûki Kaji) and his cultivate sister Mikasa Ackerman (Yui Ishikawa) witness something horrendous as the city dividers are crushed by an epic titan that shows up out of nowhere.