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. 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, others follow you incessantly, and the hated "Baiter" that appears if you take too long to finish is still here.
The major difference from the original is that it has infinite levels, whereas Defender has only five. Another major addition is the Stargate, a small, boxlike portal that transports you four levels ahead for a better challenge.
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".