The game set in the year 2348, where a long period of peace has deprived the earthlings of their instinct to wage war. The Federated Government, greatly concerned regarding this ever increasing dangerous situation, developed a Gain Ground simulation system in the year 2348 in an effort to instigate their ever waning fighting spirit. However, suddenly without warning, the Supercomputer went berserk and took many of the citizens as hostages. In order to rescue the people, three of the bravest warriors were urgently dispatched to go forth into the deadly Gain Ground.
The goal of each stage is to either get all characters you control out of the stage via the marked exit, or to kill all enemies on the map. The latter ends the level without having to get any more characters out, but it can be quite difficult depending on where the enemies are hiding. If the timer for the level runs out before all enemies are defeated, all characters in the player's party that did not make it into the exit are lost, and if all of a player's characters are killed before they can reach the exit of a level, the game endsnote .
The game also has "Hostages" that can be rescued by picking them up and carrying them to the exit. Each hostage unlocks a new characternote , which will have a unique set of weapons. However, if all of the enemies are defeated before a hostage is brought to the exit, the character will not be obtained. Also, any character that you lose turns into a hostage, who can be saved in a similar manner. However, if you lose another character when trying to save them, then the first disappears for good. This unique mechanic means that your lives are directly tied to the tactics you have to choose from- if you lose a character and cannot rescue them, you're down a life AND you've lost the weapon they had.
Several stages in Project X Zone are modeled after this game, and use a remix of its main theme.
This game provides examples of:
- Aerith and Bob: Some characters have very distinguished and unique names, such as "Zaemon", "Gascon", and "Mud Puppy". Others, however, can be as simple as "Johnny", "Betty", and "Honey".
- A.I. Is a Crapshoot: The Gain Ground system is the Big Bad, being a computer program that took the scientists who created it hostage, along with the several rescue teams that failed before yours came along.
- Alliterative Title: Gain Ground
- Denial of Diagonal Attack: Played with and averted. All characters can fire in all eight directions with their normal attacks, but some characters can only shoot in certain directions with their special attacks.
- Difficulty Spike: The first couple of Epoches are tough, but managable. The third one, however, introduces regular mooks with fast-firing, straight-shooting projectiles, and the game becomes utterly brutal.
- Easy Level Trick: Zaemon's tornadoes spiral in a circular pattern and can hit multiple enemies. While using him in most cases is a matter of precise skill and/or random luck, his magic has one major exploitation: the tornado can hit the same enemy multiple times. Therefore, one can defeat multiple-hit enemies (such as the tank boss in the fourth round of the Genesis version) by simply firing a tornado close-range at them (or in the case of the tank, in it's path) and watching it bounce around the inside of their hit box until they die.
- Easy-Mode Mockery: Playing on easy mode in the Genesis version will cause the game to skip the entire fourth worldnote .
- Empty Room Until the Trap: The enemy count at the top of the screen tells you how many enemies are left alive in a level. This is crucial, as in multiple situations, enemies can charge in from off-screen and ambush you if you don't realize what's going on. An especially common occurrence in Round 1 Stage 3, where new players can get confused after they defeat everyone and the level doesn't end, resulting in them going for the exit and being charged down a narrow corridor by fast off-screen enemies from above.
- Final Death: The game's hard mode starts you with all characters. If one dies, they'll be Killed Off for Real!note
- Instant-Win Condition: You can clear a level by either getting everyone through the exit or killing all enemies. No matter how many characters still have to reach the exit, the level ends instantly if all enemy troops die.
- Justified Extra Lives: Every life is its own character, with different attacks and range. You gain new lives by rescuing characters in the field (by touching them and then bringing both characters to the exit); if a character is killed, they'll be frozen on the map, and another character in your control can rescue them. (If a character dies while escorting another, though... the escortee will be lost!)
- Loads and Loads of Characters: Twenty unique warriors in all. Each is split into categories with different weapons. There are two spear throwers, two archers, four gunmen, four magic knights, four heavy weapon wielders, two grenade-launching characters, and two boomerang-wielding characters.
- However, in normal mode, it is possible to find duplicates of characters you already have, expanding your roster to over thirty soldiers, assuming they all survive to the end. Note however that choosing easy mode skips the modern era, loosing a few extra ones in the process.
- In normal difficulty, there is a total of thirty hostages to add to your party in single player, and forty-five in two player.
- Notice This:
- Hostages blink in different colors, are not moving or animated at all (until you pick them up), and are much smaller than any other enemies in the game in order to indicate they are important things to collect and not mooks.
- Exit(s) are giant yellow rectangles with the word EXIT written on them in bold text. Enemies refuse to stand on them, and they are almost always placed on a contrasting background, especially when they're anywhere but the entire top edge of the screen.
- One-Hit Kill:
- Every one of the twenty different player characters goes down in one hit, no matter if it's a throwing knife, grenade, fireball, a futuristic laser, or just an enemy bumping into you.
- Most enemies can be defeated in one shot, especially in the first round. In fact, some weapons are strong enough to take out enemies in one hit that would normally take multiple shots, most notably Mud Puppy's plasma cannon.
- One-Hit Point Wonder: All player characters die in one hit. See One-Hit Kill above.
- Point of No Continues: You can't continue anymore once you reach the Final Epoch; if all your characters die by that point, your game will be permanently over. To make things even worse, no extra players can join in once the Final Epoch starts, meaning that they cannot cheat in an extra play for you that way either.