The plot revolves around the creation and usage of Stargates, to travel and protect the area better. It is also the first (and only) time the Irata's name is revealed, giving players a better image of who they're fighting.
Defender Stargate is similar, yet different from the original Defender game in several ways. It is still a 2-D sidescrolling arcade shooter where you defend colonists, but the graphics have changed. The alien Landers look more like spaceships from old 1950s movies, but they still capture colonists and mutate if they reach the top of the screen. They also became nearly twice as wide as they originally appeared on Defender. The front of the Baiter ships turned silver, plus the player’s ship itself was also larger, and the animation of the planet being destroyed if all the colonists were absorbed into Landers was given a bit of a makeover as well.
Various new enemies, mostly Irata, a cousin species of the Manti, have made their first and only debut to make your life miserable. They also serve as distractions for the Landers. Some fire homing mines (known as Space Hums), such as the Dynamos if they were not dealt with immediately, others follow you incessantly, and the hated "Baiter" that appears if you take too long to finish is still here. Several other enemies were also introduced that had behavior similar to Baiters, being the Phred, Big Red and Munchies, as they were small, moved quickly, were hard to hit, and came out when the player took too long to finish an Attack Wave. Munchies were ejected from the Phreds.
The other new enemies included the Yllabian Space Guppy, which was small, moved at a medium speed, and fired several shots in a short amount of time. Firebombers moved slowly, mostly staying at the top or bottom of the screen for the most part, although when the player attempted to shoot one, they would fire a Fireball right at the player’s ship, making the Firebomber difficult to destroy.
The major difference from the original is that it has infinite levels, whereas Defender has only five. There were also special waves included that were not on Defender, such as the Yballian Dogfight, occurring with every Attack Wave ending in the number five (Attack Wave 5, 15, 25, etc.). Like with a Mutant takeover where all Landers were changed to Mutants, these waves took place in deep space (without the planet), where all enemies were Yllabian Space Guppys and a lone Pod. Similarly, a Firebomber Showdown occurred with every Attack Wave that ended in the number 10 (being Attack Wave 10, 20, 30, etc.), where all enemies were Firebombers, although on these waves the planet is left intact, however.
Another major addition is the Stargate, a small, boxlike portal that served several functions, depending on the situation: one, it would teleport the player’s ship to another area of the planet when entered (similar to Hyperspace being used). If the player entered the Starbase while a colonist was in the process of being abducted by a Lander, the player would be teleported to the abduction area, and if the player entered the Stargate while carrying four colonists, it would warp them four levels ahead for a better challenge and give them a bonus of 2000 points for each colonist that was still living. However, this special warp could only be done twice.
Inviso was also added as a defense, making the player’s ship invincible, but also invisible at the same time. A small amount of Inviso was earned with every 10,000 points scored. Also, the game had the option of giving players twice the amount of ships, Smart Bombs and Inviso at the start of a game if two credits were deposited into the machine and the one player option was chosen, which this feature was not available on Defender.
Unlike the original Defender, this game was not nearly as popular. It didn't generate the same "Defender Fever" that made the original famous. People were setting up tents in front of Defender machines, but the sequel did not sell as well. Many Defender fans thought that the sequel was ridiculously hard, so it never generated a "Defender Fever".