From the Arcade flyer:
"A long period of peace has deprived the earthlings of their instinct to wage war. The Federated Government, greatly concerned regarding this ever increasing dangerous situation, developed a GAIN GROUND simulation system in the year 2348 in an effort to instigate their ever waning fighting spirit. However, suddenly without warning, the Supercomputer went berserk and took many of the citizens as hostages. In order to rescue the POWs, three of the bravest warriors were urgently dispatched to go forth into the deadly GAIN GROUND."
I never played the arcade version but I sure put in some time on the Sega Genesis home system port of it. The final boss you face is kind of infamous in that it doesn't really take any true skill to lay him low, it just requires you send almost all of your time rescued characters into a suicidal run at him with bows, spears, machine guns, and shotguns blazing.