Emerging from his many battles both a warrior of legendary proportions and a battle-weary man, Jason starts life anew in a distant land. But one day, unexpectedly a stranger arrives at his door, and makes a plea: "I want you to capture - no, kill the creature of darkness we have succeeded in producing.

The research facility she spoke of was located in the depths of a subterranean installation in a silent forest, undisturbed by human habitation. There it had been created, an amalgam life form, an assimilation of life and machine, of multiple organic species, whose base structure was a tissue sample. A tissue sample harvested by the lab from the subterranean life form that Jason was supposed to have destroyed.

Exploiting its own astounding reproductive powers in the region underground forbidden to the human race, the life form was apparently hollowing out subterranean niches in the image of its old haunts. 

The decision to reactivate SOFIA_III seems to Jason nothing more than an inevitability.

"Back into the cold - after all." 

For SOFIA_III as for himself, war is the only design and destiny... 

(storyline from Blaster Master: Enemy Below game manual)