The ORN Empire has built a large asteroid fortress called Dyradeizer, which is rendered invisible due to shield generators hidden in various locations by ORN. The Galactic Federation, in an attemp to destroy the Dyradeizer, sends the Fire LEO spacefighter to locate the generators and destroy them, as well as the gigantic fortress. The Fire LEO disables the generators and take down the Dyradeizer.
Thunder Force II
After the failure of the Dyradeizer, ORN builds a powerful new battleship called Plealos. ORN utilizes Plealos to attack the Galactic Federation, successfully destroying the planet Reda and taking over the planet of Nebula. The federation discovers that ORN is hiding Plealos beneath Nebula's surface, so they prepare an offensive operation, sending the Fire LEO-02 "Exceliza" to destroy Plealos. Exceliza takes on ORN's defenses in the planet and reaches Plealos, which had just took off, so Exceliza pursues it and finally destroys it.
Thunder Force III
A century after the last battle against ORN, the Galactic Federation had been trying to spot ORN's headquarters, with little success since ORN installed cloaking devices on five planets on their territory and created a defense system in the form of Cerberus, a battleship specially designed to neutralize large ships and fleets. To counter Cerberus, the Federation develops the Fire LEO-03 "Styx", which was small enought to avoid detection but with vast firepower. Styx is then sent into battle, destroying the cloaking devices and Cerberus. With the location of ORN headquarters revealed, Styx infiltrates it and finally destroys Khaos, the central computer controlling ORN's forces.
Thunder Force IV
The ORN Empire was thought to be defeated, but even so the Galactic Federation was still suffering attacks from hostile forces. They eventually find these attacks come from Vios, an ally of ORN who had joined with ORN's residual forces and were growing in strenght. They find Vios headquarters in the planet Aceria, sending a fleet to attack, but they end defeated. They then create the Fire LEO-04 "Rynex" to destroy Vios forces. Upon taking over various key planets from Vios, Rynex faces off against ORN Faust, but as Rynex is joined by a fleet of Styx ORN Faust retreats to Vios base. Rynex gives pursuit and reaches Aceria, where it fights against Faust again, this time apparently destroying it, as well as taking down the reconstructed Khaos. In the ensuing explosion, Rynex is sent drifting into space, but the pilot safely ejects. Before ejecting, the pilot discovers Faust's plans to attack a distant planet called Earth, so he leaves a message on the ship, hoping that the Earthlings could find it and prepare to fight ORN's forces.
Thunder Force VI
In the year 2161 AD (ten years after the fateful battle against the Guardian supercomputer) the ORN Empire finally reaches Earth, launching a full-scale attack under the leadership of ORN Faust. Earth is powerless against the alien threat, being easily overwhelmed. Suddenly, strange readings are picked from the island of Babel, where the remaining Vasteel technology (technology created after analyzing Rynex) was sealed away, apparently reacting to ORN's approaching. As researchers study the remaining fragments, they finally find the message on Rynex, which also contained coordinates to the Galactic Federation homeworld. Earth's scientists, then recreate the Vasteel technology, building the RVR-00 "Phoenix" to fight against ORN's forces. The Phoenix takes on the invaders, destroying the rebuild Cerberus battleship and reaching the capital of the Galactic Federation, where a squad of Styx was fighting ORN's forces, joining them in the fight. The combined forces of the Galactic Federation and the Unified Earth Fleet then attack the ORN Empire at full force, destroying ORN Faust and the revived ORN Emperor Khaos, putting an end to the ORN menace at last.