A century has passed since the hellish war between our Galaxy Union and the Empire of Lone broke out, and it has spread throughout almost half of the galaxy.
Our Galaxy Union finds itself in a precarious position as its armed forces are diminishing under the Empire's massive weapons. The Empire has also succeeded in hiding its planet by installing the HEXA-CLOAKING- SYSTEM on its main base and on its five major planets (HYDRA, GORGON, SEIREN, HAIDES and ELLIS). Thereforce, in the Lone Star System, we can neither approach nor destroy anything from ordinary space. Also, the enemy has been using a great amount of RIS (Remote Intercepting System) called KILLPAROS, to surround their star system. Their RIS has never failed to destroy our attacking star fleet divisions. We are about to face the hardest mission we have ever experienced. Project Thunder Force III is our final challenge. The starfighter FIRE LEO 3, code name STYX is the key to victory. STYX has a small body that cannot be detected by the KILLPAROS, but still has the capability of carrying many types of the extremely potent weapons that the larger starfighters have.
STYX is now ready for the mission.
--From the Genesis Thunder Force 3 instruction manual.