The tumultuous 2030s.
The world faces a time of chaos due to the waves of military invasions led by General Morden.

So far, the dictator General Morden-"Satan's Stand-in"-plots to conquer the world and instigates a series fierce battles by starting coups against the Government Forces, stealing its all-purpose vehicle METAL SLUG, and allying himself with unknown life forms.
But at every turn, thanks to the crack performance of four agents-the pride of the Government Forces-Morden's ambitions were shattered to pieces.

Worn down by this series of defeats, the fighting power of Morden's Forces was reduced to mere remnants scattered throughout the world and holdouts in the city of their main base.
Morden had been rendered militarily impotent.
But just when everyone in the Government Forces thought Morden neutralized, reports concerning the discovery of hidden Morden Forces from the Division of Intel comes racing across the wires.
Word has it that a mid-sized detachment is concealed in a mountain village not too far from the Government Forces' main base.
In response to this, GHQ judges the information to be highly credible and directs the special-ops unit Peregrine Falcons to take a second look.

Receiving their marching orders, Marco and Tarma quickly catch a ride on a transport helicopter and head for the site.
Tarma optimistically views this as nothing more than another SNAFU.
But Marco can't help feeling Morden is once again on the move, because his trigger finger is getting mighty itchy.