The setting and events of Nocturne take place in a separate continuity to the rest of the Megami Tensei series. Nocturne is set in modern-day Tokyo before and after an event called the Conception, in which the normal world is destroyed through the actions of a cult to prevent the world's stagnation and ultimate demise, as dictated by the Great Will governing existence. Tokyo is transformed into a wasteland on the inside of a sphere inhabited by the spirits of humans who died in the Conception; demons who wage war with each other; and Reasons, deity-like beings formed from the gathering of Magatsuhi energy by people with a deity or demon sponsor. Those who possess Reasons seek to remake the world in their image by presenting themselves to Kagutsuchi, a semi-sentient manifestation of the Great Will's power which exists to perpetuate the destruction and rebirth of worlds through Conception, and proving their worth.
The player controls a silent protagonist who is given a name during the opening. The player navigates the Vortex World through both a 2D world map, and to-scale 3D environments used in dungeons. While navigating the world map, the Protagonist's party and other non-player characters (NPCs), interactive objects and landmarks are represented by symbols: these objects and NPCs include containers holding items, spirits that will heal the party, and a globe in the bottom left-hand corner of the screen showing the party's approximate location on the world map. During dungeon navigation, the Protagonist navigates grid-based, multi-leveled areas. Different areas are accessed via doors, and as the Protagonist progresses the area is automatically mapped. While exploring dungeons, the in-game camera can be freely rotated, and a "Look" action allows the player to see environments in first-person. The game world can be navigated either on foot or by using a fast travel system unlocked at a certain point in the game.
Source: Wikipedia, "Shin_Megami_Tensei_III:_Nocturne," available under the CC-BY-SA License.