The player is thrust into modern-day Tokyo trying to survive a demon invasion with the knowledge that most of the people around them will not live longer than 6 more days. The government has sealed off the subways and roads to keep the invasion from spreading, with a cover story involving poison gas let loose in the system.
Exploring Tokyo
Exploring takes place primarily through a menu, with a map on the DS' upper screen showing the location of each prefect as it is selected from the menu on the lower screen. Each area will give a variety of options, based on the time and day selected, but will usually have the options to Look around, Listen to people here, and Talk with your party. These three options will generally tell more of the story, but aren't usually required to advance the story in any way. When events are available in an area, such as battles or chances to talk to other characters, they are listed here along with the previous options, and usually marked with an icon (which will also be displayed on the list of locations). A clock icon is displayed for events that consume time.
Generally, time will advance towards some pre-determined event, usually foretold as doom for the party. The characters receive emails telling them that some events are going to occur during the coming day, usually involving the deaths of several people and/or the coming of some major demon. The main character also sees a number over each person's head which tells him how long they have left to live (in days), including the characters in the party. This means that while there is some freedom for the characters to move about Tokyo and do as they please, they are limited in the sense that their deaths are hanging over their heads in the form of a number counting down.
The COMP
While exploring, pressing 
will bring up the COMP menu. The COMP allows the characters to manage and summon their demons, and communicate with other COMP users. While the options are initially limited to allow the characters some time to learn the new features, eventually they have access to the following menu options: Team, Fusion, Profile, Auction, Mail, Data, Config, and Back.
Team
This menu allows the characters to manage their party. Each character can take 2 demons into battle with them, and up to 4 characters can go into battle together. The demons not currently in a party are displayed in a list on the right side of the screen, while the parties are displayed on the main portion of the screen in a 3 x 4 grid. Each character's skills can also be set here (by pressing 
when the character is highlighted).
Fusion
Fusion allows the characters to create new demons by fusing together two demons. The level and skills of the new demons is determined by those of the demons used in the fusion, while the type of demon is determined by the types of the demons chosen. The results of such fusions can be found in the section on Demon Fusion. This is easily one of the most important features for allowing the characters to survive.
Auction
Once characters begin to acquired currency usable in the demon world, they gain access to the Demon Auction. Here they can choose from a list of demons and bid (or buy outright) on a contract with that demon. If they win the auction, they can use that demon in battle, or in fusion to gain new demons. New demons become available for auction as they are discovered elsewhere in the game, or as the characters gain more experience in the auctions themselves.
Profile
This menu gives some basic information about each of up to 12 major characters that have been met so far in the game. This will include name, age, handle, and a short description.
Mail will give more information on how to use the system as well as important events that have occurred (or will occur). It also allows some communication with other characters that have access to the COMPs.
Data
The Data menu allows the player to save their progress, though this game only has one save slot.
Config
Here the player can set various options: Message speed (1-3), Message skip (allow the player to use 
or 
to skip through messages), Display help (on-screen display of the button usage during most portions of the game), and Direction (which direction a character will go when pressing 
on the battle grid).
Back
Close the COMP and return to the exploration menu.
Battle
Battle takes place on a grid displaying the area in which the battle takes place, the leader of each party (up to 4, and not the demons that are in each party), and the leaders of each of the demon parties in the area.
At the start of battle, the player is given the chance to select skills that each character can unlock from various demons on the map. Only that character can unlock that particular skill by defeating the particular demon chosen once the choice is set, and it cannot be changed during the course of the battle.
When one group attacks another, the game shifts to a first-person battle view displaying the (up to 3) enemies and status information for everyone involved in the battle (the enemies across the top, the player's party across the bottom). The player's party can not be seen on the screen other than the status icons displayed across the bottom (again, it's a first-person view, more or less). Each character's actions are chosen from a menu at the beginning of the battle, and one round of combat occurs. When finished, whether all of the characters on either side are defeated or not, the game shifts back to the grid view for the next party's turn.
At the top of the (lower) screen in the grid view, the turn order is displayed for all of the characters in the area (both the player's characters and the enemies). Actions taken on the grid determine when the character will have their next turn. Moving, using a skill, and attacking each delay the next turn by a specific amount, and this is shown by an arrow on the turn order display.
In the attacks themselves, each character has a chance to gain another turn, which will allow them a second attack before returning to the grid view. It is important to exploit the weaknesses of the enemies in order to gain this extra turn, as well as preventing them from gaining it. Critical hits can also take away (or give) an extra turn. Using a spell or skill that a character is strong against can result in that character gaining an extra turn (or the character using the spell losing one).