Shingeki no Kyojin (Attack on Titan, which at least in French is correctly translated as ''L'attaque des Titans'', AKA ''The Titan's attack'') is an anime manga with the simple but efficient premise of ''giant things eating humans''. I think chapter one laid out the quest very well :
''Eren (young hot blooded teenager with gnarly latent powers), return with presumably your adopted sister Mikasa (Japanese girl in a setting where everyone have german names, and adopted sister to allow that traditional incest vibe...) in the basement of our home, in the area that just got invaded by the said gigantic things eating everyone''
85 chapters later, Eren does reach the basement (with a lot of things happening-maybe too much)
What follows will not make any sense for people not following AOT. For the others....
1)It was a damn picture in the basement
2)That picture triggers a very extensive flashback
3)The world is not dead beyond the walls.
4)The walls are set upon an island, ''Paradise''
5)Beyond the island, the world is free of Titans-and have at least WW1 technology-biplanes and dreadnought.
6)The denizens of Paradise ruled a global empire, but kudos to the creator to at least acknowledge this, in the''totally not China'' mainland, the future denizens of ''island of the coast exactly in the position of Japan toward China'' committed an heaping of warcrimes using their Titan powers. Big war. Vanquished. Usual stuff.
7)The people of ''Paradise'' are either now caged on ''Paradise'' island, or in death camps on the mainland
8)I have to admit that I lost the plot there, but Eren parents are either Mandchurian (pun intended) agents of the mainland power, or resistant to the mainland power, or ''B while pretending to be A''-the French scanlations are horrible
9)Paradise is akin to a North Korea regime, with the rulers threatening to unleash WMDs (Titan-the thousands of titans inside the walls) if anyone lands on Paradise island, the last bastion of their power. That security system strike me as very crummy, mind you, but...
That's nice, but maybe he could have just told him