The year is 2120: Earthlings have spread beyond their galaxy and have started to colonize uninhabited planets in different solar systems. One such planet to be colonized was the Earth-like planet Gloria which happened to have an enormous natural supply of gold. This discovery became pivotal to Gloria's emigration, so much that emigrators would be impoverished by the cost of reaching the planet. Because of this, life on the planet thrived in a setting similar to the American Old West. Though poor, the gold trade aided the Glorian's in advancing technology and knowledge, so much that among the inhabitants lived talented inventors and engineers.
However, the Glorian's were not the only ones who were tempted by the planet's gold: Space pirates known as the Wild Lizards quickly invade Gloria and decimate the towns of the people and enslave those who survived in an attempt to deplete the planet's gold supply.
Two inventors of Gloria who were a part of the planet's development teams decide to strike back at their invaders using two fighter planes in the shape of giant revolvers with plane wings.
Source: Wikipedia, "Gun Frontier (arcade game)", available under the CC-BY-SA License.