Costa Rica is the main setting for Big Boss’s mission in Metal Gear Solid: Peace Walker.
The game begins in 1974, when an unidentified military force appears in Costa Rica. The country has no standing army, so the crisis draws the attention of the Militaires Sans Frontières, the private military organization led by Big Boss. Their investigation eventually uncovers a nuclear threat and the development of advanced weapons.
Costa Rica's neutral status is important to the plot. Because the country does not maintain a conventional military, its government seeks outside help while trying to avoid becoming a battlefield in the Cold War. The game's locations include tropical environments, military facilities, and other Central American settings.
Peace Walker later expands beyond Costa Rica, including missions connected with Nicaragua. That can cause confusion, but Costa Rica is the initial and primary national setting associated with the game's central crisis and opening operations.