People chased by giants hiding in train

When man-eating Titans first appeared 100 years ago, humans found safety behind massive walls that stopped the giants in their tracks. But the safety they have had for so long is threatened when a colossal Titan smashes through the barriers, causing a flood of the giants into what had been the humans' safe zone.

100 years ago, titans suddenly appeared on Earth. Soon, human civilization veered on collapse due to the titans. Humans then built a giant wall to defend themselves. Within the giant walls, humans lived in peace, but, 100 years later, the giant wall is broken.

A subway train is suddenly attacked by mysterious monsters.