The game takes place in a fictional version of the Shibuya shopping district in Tokyo, Japan. While everyday life goes on in the Realground (RG), the chosen dead are brought to an alternate plane of existence called the Underground (UG). The UG is also the venue for the Reapers' Game. By offering their most treasured possession to enter the Game, the dead gain the chance to contest for the prize: to be brought back to life or to transcend to a higher form of spiritual existence. Most of those who choose to transcend become Reapers, the opponents of players in future Games. A player in the UG is invisible to the living in the RG, though one can sometimes read and influence their thoughts. The UG is frequented by creatures called "Noise", which are attracted by the negative feelings of the living. To progress in the Reapers' Game, players are often required to defeat Noise by killing or "erasing" them.
The game's combat system is called the "Stride Cross Battle System". The combat takes place across both screens of the DS, with Neku on the touchscreen and his partner on the top screen, representing the different "zones" of the same local area; the two characters battle the same enemies that exist in both "zones" simultaneously. Neku and his partner are synchronized during battle; they share the same health bar so that even if one character does not take any damage, the pair can fail in battle if the other takes too much. A green "light puck" will pass between the characters during battle; by alternating battle between the character who possesses the puck, damage is increased. The movement of the light puck is determined by the "sync ratio" between Neku and his partner; the puck stays longer with the character with higher ratios. The player can equip Neku and his partner with clothing that can alter the light puck's speed. The light puck also can magnify Neku and his partner's attacks as long as when they hold the puck, they are not attacked by an enemy and can continue this in a volley.
Source: Wikipedia, "World_Ends_with_You," available under the CC-BY-SA License.