It's about these humas who are living inside a walled city. The walls are there in order to protect them against a Titan/Giant invasion that happened 100 years back. Almost all of humanity got wiped out back then. But on one day an incredibly huge Titan appears and he breaks one of the gates which will lead to yet another Titan invasion and the fall of Wall Maria and the Shiganshina district in which Eren, Armin and Mikasa grew up. Eren then vows to kill each and every one of these titans, Armin and Mikasa follow him in his path of revenge.
I'd watch the anime first for this one, it's only 8 episodes in. After that you could pick up around chapter 9/10. And the art in the manga.... it's difficult to get used to, but the plot is worth the try.