Shin Megami Tensei IV: Apocalypse is a follow-up to Shin Megami Tensei IV, taking place in the same setting, and with its plot acting as a follow up to the Neutral path of the previous game.
The story is set in the year 2038, in Tokyo, where a raging war occurred between the gods 25 years ago, resulting in the sky above the city being covered in rock, in order to avoid contact with the outside world. Even after the war ended, the world is still devoid of normality, with people killing each other, and Man being ruled over by both angels and demons.
The player takes control of the character Nanashi, who after being killed almost at the start of the game, is revived by the demon Dagda, in return for becoming Dagda's "Godslayer".
The game then slowly directs the player through the usual Shin Megami Tensei alignment system, where decisions made will border on either Law, Chaos or Neutral routes, which will lead to a multitude of different endings.
There are several key gameplay changes in this game, comparing it to its sister title, Shin Megami Tensei IV:
The expanded partner system is the most obvious improvement, with Nanashi being able to have up to seven partners during battle (the usual 3 demons to fight alongside him, as well as 4 support characters, each with their own support gauge);
Update to the negotiation system, where previously failed negotiations will start where they left off if you try them again, instead of simply restarting all over;
Enhancements to magic usage, whereby demons specializing in a certain magic type will use it more efficiently;
Also regarding magic, the light and dark spells of Hama and Mudo no longer have the possibility of instantly killing an opponent, they now deal damage instead;
Complete overhaul to the UI, making it a lot more user friendly and aesthetically pleasing;
New sound effects (like demon roaring).
Some demon designs from the previous game were refined in this release, and the main character was given a teenager look in order to better fit the game's themes (instead of the original design, which made him look much older).