Agent Dark Widow, guardian of the Shield of Invasion, has fought endlessly against the alien group Eltron whenever they threaten Earth. However, the tragedy etched deepest in her heart was the death of her comrade, Nana. To uncover the killer responsible for Nana's death, Dark Widow heads to the site where Eltron appeared. But the location has become a battlefield under Eltron's assault. Battling through waves of aliens and leaving many wounded, Dark Widow survives—only to face the boss standing before her: Deltzer Alien, the true identity of Nana's murderer. Fueled by rage and vengeance, Dark Widow attacks Deltzer Alien... but the outcome takes an unexpected turn into a BAD END.