In the darkest reaches of infinite space, pure evil lies in wait ... The Dark Forces led by the god of destruction, Bacterian, prepare to launch a massive onslaught against the planet Gradius. Possessing unstoppable power, the Dark Forces threaten to plunge the Universe into total war and complete annihilation.
Their invasion force smashes the Gradius defenses, and the surrounding planets soon fall like dominoes. Now Bacterian's evil grasp reaches for the heart of the resistance...planet Gradius itself...
To counter the oncoming threat the United Gradius World Forces gather all their fighter craft. Half of them form a defensive net - the other half a strike team. But they are badly outnumbered and The Dark Forces easily tear through their thin frontline. No fighters survive the slaughter....
In a last gasp move, the United Gradius World Forces send out two fighters, previously considered too dangerous for combat missions: The InterDimensional VIC VIPER fighters.
The Fighters take off for the heart of darkness to join the fray that will decide the fate of the Universe...
There are a total of ten levels in the game. Stage 4 is notorious for being the first and only pseudo-3D level to ever appear in the Gradius series. In this mini-level, the player controls the Vic Viper in a third-person (behind the ship) perspective and must avoid colliding with walls. Though the level is completely devoid of any enemies, free floating power-ups are scattered throughout. There are also two hidden levels that are based on the early sections of Gradius and Salamander. The game contains the familiar weapons, level layouts, and enemies that have become trademarks of the series.
The game is known by fans as being considerably more difficult than its predecessors, so much so that it prompted Konami to pull it from arcades rather quickly. (The arcade version did not provide a way to continue the game upon losing all lives, and did not even include an operator-selectable "allow continue" option.) The Japanese version of the game contains a 'beginner mode' that allows the player to venture through the first three levels at a much easier difficulty. At the end of the third level, the game ends immediately and bids the player to try the game again at the normal difficulty. The Asian arcade release lacks the beginner mode and retrospective introduction sequence, but reduces the difficulty overall.
The biggest addition to the game is the introduction of the "Edit Mode", which is a logical progression on the weapons system from Gradius II. Not only can players choose between pre-defined weapon schemes, but they can mix and match missile, double, laser, shield and "special" ("!") power-ups into their own custom combination. However, some of the weapons available in pre-defined schemes can not be used in custom schemes, and vice versa. Many of the redundant weapon variations from the different schemes in Gradius II were removed, in favor of new variations.
A port of Gradius III was released for the Super Famicom in December 1990 and the SNES in 1991 with the option of reduced difficulty and additional armaments for the Vic Viper. It replicates the slowdown of its arcade counterpart and discards the pseudo-3D and Crystal levels. It also introduces a boss called Beacon which awaits the player at the end of the new high-speed stage, which is a counterpart of the high-speed stage in Gradius II. Unlike any other version, the Super NES port is the only one that allow players to continue when they lost all their lives. Also, there are bonus areas that can be accessed by finding an entry point and fulfilling certain conditions. A harder difficulty called "arcade" can be unlocked by inputting a code on the options screen, however, it is simply the same SNES game at a harder difficulty, and not an accurate port of the arcade version.
In addition, most stages (and some bosses) from Gradius II will appear after the second level.
This port was also released for Virtual Console on April 23, 2007.
In 2000, Konami bundled Gradius III and Gradius IV Fukkatsu together for release on the PS2 video game console, as Gradius III and IV. The port is based on the arcade version and has an unlockable Extra Edit mode, which gives the player the freedom to create a weapon array from all included setups and adds the F-Option, R-Option and Reduce II power-ups found in the Super NES port. The Reduce from the SNES port returns the player one step closer to the Vic Viper's original size when hit, giving it protection from two hits.
As the PlayStation 2 is technically more sophisticated than the game's original arcade hardware, the game as a result runs faster in situations that would normally impose lag. KCET implemented a "WAIT LEVEL" regulator as an option that can be adjusted from three levels at any point in the game, beginning from 0 to 2 (original rate).
While the PS2 version doesn't have the option to continue as the Super NES version, it features the possibility to select any stage the player has cleared.
Gradius III was later ported to the PSP in 2006 as part of Gradius Collection. This version keeps the tradition of not allowing the player to continue after exhausting all reserve ships. However, the player may save their progress at checkpoints using the PSP's memory stick.