Metal Gear Solid: Peace Walker is the final entry in Konami's tactical espionage series for the PSP, and the sequel to 2004's Snake Eater. Players take the role of of the legendary soldier Big Boss in the 1970's, where he leads a band of mercenaries stationed in Columbia, South America. Big Boss is called to action in Costa Rica, where a mysterious armed force is threatening the peace in a country that has remained neutral following the Cuban Missile Crisis.
Peace Walker revolves around infiltration missions, divided into those which must be played in a set order for the purposes of the game's plot, and optional missions with more challenging objectives. As in previous titles, players are rewarded for prioritizing stealth and non-violence during missions. Unlike previous Metal Gear titles, players can choose to undertake certain missions as a soldier under Big Boss' command instead. Peace Walker includes Co-Op and competitive multiplayer modes.
The title also features a simulation mode in the form of building and maintaining the Mother Base, an abandoned OTEC research platform re-purposed by Big Boss to serve as his army's headquarters. Players recruit and manage the morale and well-being of up to 350 soldiers and staff members. Enemy vehicles can also be captured and brought back to the Mother Base, and new weapons and items for use in missions can be researched and produced on-site. Recruited soldiers and captured vehicles can be used in special "Outer Ops" missions against CPU opponents, where success will increase their stats as well as generate income for the Mother Base.