Players take the role of a nameless high-school student who attempts to defend Tokyo from an invasion of demons that threaten to turn the city into the next battlefield for the endless struggle between the forces of Law and Chaos. There are a variety of demons based on several religious mythologies, and each family has its own racial alignment. Players will develop a Lawful, Chaotic or Neutral alignment over the course of the story, depending on the actions they take and the demons they befriend or fight. Different alignments will result in entirely different endings for the game, all of them equally valid - although the sequel assumes the player completed the Neutral ending.
The story begins, after a strange dream sequence, with the discovery of a computer program e-mailed to the protagonist by a mysterious ally, allowing him to communicate with demons and summon ones allied with him via his hand-held computer, or COMP. Shortly afterward, missing persons are reported, fellow students go missing, and the protagonist is attacked by the demons which he so conveniently now has the capability of conversing with and summoning. Attempting to unravel the reasons behind all of this (and save Tokyo) is what makes up a majority of the story.
Movement is split into two areas. When outdoors, the game presents an overhead view of a map of the local area, and the player can move the icon representing the protagonist around the streets and into buildings. Random encounters with demons begin to hinder your movement here after an early plot point. W hen indoors, all movement takes place in a first-person view remeniscent of old PC RPGs like Eye of the Beholder.
Encounters with demons begin with an opportunity to attempt to talk before fighting. Depending on what the player says, the demons may decide to fight, run away, offer a bribe, or join the player's party and become an ally. What demons players battle and ally with will partially influence their alignment, which will affect how other demons react to them in the future. Demons whose alignment oppose the player will never join their party. Alignment can also affect what items can be equipped and what bosses are fought.
Once combat is initiated, you select actions for each of your party members to take. Turns occur simultaneously for both sides, with each combatant taking their action in the order of initiative as determined by their speed and other similar factors. The protagonist cannot use any spells, but most of the other characters and demons have some form of magic at their disposal. Killing a demon nets you some experience and money, and each time you gain enough experience to increase your level, you can select a character stat to increase. Although spells are specific to each character, all other statistics are determined by you during the dream sequence, which means you can decide whether your team consists of glass cannon wizards or melee bruisers as you like. Players should take note, however, that this is an Atlus RPG and hence isn't very forgiving.
There are also some stores in the city where you can purchase weapons, armor, or usable items as needed. A bit into the game, you can purchase some additional items with more useful traits, although they are very expensive.
Demons are ordered from highest level to lowest. Races are organized by affinity (Light-Neutral-Dark) and alignment (Law-Neutral-Chaos). Demons marked with (MCD) only appear in the Mega CD version of the game.
Translation into English
The full translation patch for Shin Megami Tensei was released by Aeon Genesis on June 30th of 2002. It includes a translation of the game manual which is virtually necessary to play the game.