Taking place directly after Thunder Force III, the ORN Empire is thought to be defeated by the Galaxy Federation, but still suffers from increasingly frequent attacks from hostile forces. The forces are discovered to be the “Vios”, an army made up of allies and residual forces of ORN. The Galaxy Federation discovers the location of their headquarters on the planet Aceria and attacks, but since the power of Vios has grown greater than the previous ORN Empire, the Galaxy Federation forces are initially defeated. Once again, they develop a new small yet powerful fighter spacecraft, the FIRE LEO-04 Rynex to eliminate Vios. The player controls Rynex and travels through ten stages while battling Vios forces.