According to the game's creator Hideo Kojima, Metal Gear Solid V: The Phantom Pain will actually be split up into two interdependent stories: Ground Zeroes, which is a prologue set in 1975, and The Phantom Pain itself, which takes place in 1984. This somewhat unusual decision was made to make things less confusing for players, who Kojima says might be confused by the open world of The Phantom Pain, which is a new feature to the series.
"I think if players of the previous Metal Gear Solid games are immediately thrust into the open world of the main game, they might find it confusing." Kojima explained. "Simply put, the main game world is vast. And you have no idea from where enemies will target you. As time passes, the game's reactions change depending on the player's actions. So much so that it's hard for us on the development side."
Kojima says that Ground Zeroes will acclimate players to what promises to be a vast change in the series. While both time and weather conditions are fixed, the rest of the world is entirely open, allowing players to carry out missions when and how they want to.
"A lot of people seem to misunderstand when I use the term 'open world' – [MGSV] is not a game where Snake spends a day fishing or changes jobs and leads a different life." discussed Kojima. "The field is open world, and you are free to do what you want, but your objective in MGSV is clear cut."