Metal Gear Solid V consists of two games - a prologue game, Metal Gear Solid V: Ground Zeroes, and its sequel, Metal Gear Solid V: The Phantom Pain.
The prologue, Ground Zeroes, takes place in 1975, after the events of Metal Gear Solid: Peace Walker. Big Boss, the main character, is working for Militaires Sans Frontières, or MSF, under orders from Kazuhira Miller to infiltrate a base known as Camp Omega in order to recover two prisoners. The prisoners are Chico, a young solider, and Paz, a Cipher agent.
Something goes wrong and the base of MSF is destroyed. Big Boss is severely injured, leaving him in a coma.