Players control Snake through the game map and attempt to complete missions while avoiding being spotted by the enemy. The threat detection system is revamped, with the removal of the onscreen meter or gauge to indicate the player that they are in the 'alert/evasion/caution' state as in previous games, which have instead been replaced by visual and audio representations. Snake's binoculars have also been revamped; instead of selecting the binoculars as an item, the binoculars are hot keyed to a button. The player is able to mark enemies and be aware of their locations in the iDroid's holographic display. There are directional cues to help players when an enemy sees them from a distance and gets closer to them. A new "Reflex Mode" is activated when an enemy finds Snake and the player only has a short window of time to take out the enemy before he fully sounds the alarm. Lights can be shot out to further aid the player's stealth efforts.
Ground Zeroes features seven missions in total, locked at individual levels. After completing the main mission the player will unlock four subsequent missions (referred to as "Side Ops") set at different points in the day, all set in Camp Omega. These missions involve specific tasks, such as eliminating certain targets or obtaining classified information. After achieving a certain objective in the main mission, the player may unlock two additional "Extra Op" missions.
As the game is introductory in nature, being a prologue, the core story mission can be completed in a very short time frame. However, the game has many additional challenges, missions and alternative methods of play. A companion app for iOS or Android was made to help players access the in-game map and other iDroid functions on their smartphones. Console versions of the game have community features that allows a player to look at other players' performance in the game, even showing where they were killed on the map.
Atmospheric weather and lighting in Ground Zeroes is mission-proprietary, meaning that the setting of the game depends on the individual mission being played. A real-time day and night cycle was originally planned for the game, but left out in favor of fully utilizing it in The Phantom Pain. Kojima has spoken about the restrictive nature of previous Metal Gear titles, saying that they "set the player on one rail to get from point A to point B, with a certain amount of freedom between". In contrast, Kojima's team instead offered the player new ways of traversal and sneaking methods, such as taking a jeep, a car, or a helicopter around the mission area. This was in order for Kojima to accomplish a "true open world experience" with Ground Zeroes.
Plot
Setting
Set in March 1975, a few months after the events of Peace Walker, players control returning protagonist Snake, also known as Big Boss (Akio Ōtsuka/Kiefer Sutherland), as he works with Militaires Sans Frontières (MSF) to infiltrate an American black site on Cuban soil called Camp Omega. Once used as a refugee camp for people fleeing Cuba and Haiti, Camp Omega is home to an old prison that is being used for extraordinary rendition as it has no legal recognition under the United States Constitution. Snake's main objectives are the extraction of Paz Ortega Andrade (Tara Strong/Nana Mizuki), who has information regarding the whereabouts of the person or persons known as Cipher; and Ricardo "Chico" Valenciano Libre (Antony Del Rio/Kikuko Inoue), a Sandinista and former child soldier who was captured by Skull Face (James Horan/Takaya Hashi), the commander of paramilitary force XOF, who are in control of Camp Omega. Snake's mission is supported by Kazuhira "Kaz" Miller (Robin Atkin Downes/Tomokazu Sugita) and Dr. "Huey" Emmerich (Christopher Randolph/Hideyuki Tanaka) from MSF's Mother Base in the Caribbean Sea.
Story
In the aftermath of the Peace Walker incident, Paz Ortega Andrade is missing and presumed to be dead. Under the direction of Kazuhira Miller and Huey, Militaires Sans Frontières turn their attention to an upcoming inspection by the UN amid suspicions that they possess a nuclear weapon, which MSF believe to be an attempt by the Cipher organization to stifle them. Their preparations are interrupted when MSF agents in Cuba investigate reports that Paz survived and has been detained at Camp Omega. The situation is complicated when Ricardo "Chico" Valenciano Libre attempts to rescue her and is also captured. Believing that both of them could compromise MSF, Snake is sent to infiltrate Camp Omega and extract them if they are alive, or confirm that they are dead and if so, determine what they revealed to their captors. Snake also recognizes the value in rescuing Paz, as he believes doing so will convince Cipher to stand down while Paz will be more willing to discuss Cipher's activities with MSF.
Snake infiltrates Camp Omega as a mysterious Special Forces unit known as XOF departs. He locates and extracts Chico, who claims Paz is dead. Using a recording that Chico gives him, Snake and Miller deduce that Paz is alive, and was moved deeper into the camp. He sneaks into Camp Omega and manages to rescue Paz as well, with the group escaping via an MSF helicopter designated Morpho One. However, on the way back to Mother Base, Chico discovers that Paz was surgically implanted with a bomb, prompting Snake and an MSF medic to remove it by hand. They then arrive to Mother Base to see it in flames as XOF forces attack. Morpho One lands long enough for Snake to rescue whatever staff they can, including Miller, who claims that the UN inspection was nothing but a ruse for the XOF ambush, which destroys Mother Base. As they attempt to escape XOF, Paz regains consciousness and warns everybody that there is a second bomb inside her body. Knowing that she is about to die, she jumps out, trying to throw herself clear of the helicopter. She succeeds, but the explosion causes Morpho to spiral out of control and collide with a pursuing XOF helicopter.
The epilogue reveals that the United States government downplayed MSF's destruction and attempted to cover up their dealings with the organization, as do many of MSF's clients. There are no known survivors of the assault, apart from Snake, Miller, and Huey. In a post-credits scene, set prior to Snake's arrival at Camp Omega, Paz is being interrogated by Skull Face, the commander of XOF, who demands the whereabouts of Cipher / Zero, as Paz had met Zero in person. Paz eventually gives in and agrees to tell him Zero's location before the scene ends. By collecting audio cassette tapes around the base, it is revealed that Huey arranged for the United Nations to come to Mother Base against both Snake's and Miller's objections, prompting them to hide the nuclear equipped Metal Gear ZEKE underwater. Other recordings reveal that Paz and Chico were tortured, physically and psychologically, for information about the MSF base. Chico, after being tortured and forced to sexually assault Paz, tells Skull Face the location of the base and what defenses there are. In a later recording, Paz is heard calling Chico a "Coward" and "Traitor" for giving up the information, however in the final tape she records herself saying she forgives Chico and would not have made it through the torturous interrogations without him.
