In the years after Metal Gear Solid 3: Snake Eater, Naked Snake has cut ties with the United States and settled in Colombia, where he founds Militaires Sans Frontières — a private military outfit that lends its support to nations in need. But when an army of well-equipped soldiers seizes a defenseless neighboring country and brings with it a nuclear warhead ominously labeled "Peace Walker" as a show of force, the government turns to Snake and his Militaires Sans Frontières to save the country.