Throughout each level, there are various notes, photographs, and audio recordings that can be discovered and examined, which are then stored in a special evidence room that can be accessed at specific points during or between levels. Although the gathering of these notes is not required to complete the game, collecting 100% of a level's evidence unlocks a code that can be used to open specific doors in each level.
Combat is similar to that of most first-person shooters, although it is discouraged, with the explanation that killing people threatens the stability of the memory and could cause it to break down completely. The player's preference for either stealth or aggression also affects the ending they receive. Black has access to a small arsenal with which to defeat enemies, including pistols, assault rifles, and shotguns, as well as the option to perform a stealth kill at close quarters. One unique aspect of combat is a device known as the CornerGun. When it is equipped, the player can turn the gun at a 90-degree angle and look and/or fire a weapon in that direction using the smartphone. Combined with the game's emphasis on using cover, it allows the player to fire around corners or over the top of low walls and tables.
It is reportedly possible, though very difficult, to complete the game without killing anyone.
Puzzles consist of deciphering codes with which to open doors, or the use of valves and levers to open a particular passage. These often make use of the smartphone's apps in specific ways. Levels also contain anomalies which, when scanned with the smartphone's camera, cause the environment to alter in certain ways, usually to the player's advantage - for example, causing a van to appear in a parking garage so that an enemy's line of sight is diminished, or making a wall vent disappear, thus opening up an alternate route.
At certain points in the game the player will have to make choices that can have consequences later on. For example, an early scene has Black decide whether or not to release an asylum inmate from a cell. Later, he has a choice between rerouting steam through pipes to open a passageway, or bypassing the puzzle by shooting the lock off a door. Like their approach to combat, the player's choices are commented on by Red, and influence the ending of the game.
Late in the game, players take control of Red. While controlling identically to Black, Red does not use the smartphone or CornerGun. Instead, he has the power to assimilate enemies and take their weapons, which he can then use. He also makes use of warp points, which are in specific places in each level, to move quickly and avoid enemies. Instead of the map, he has a sonar vision that allows him to briefly see where enemies and warp points are from a distance. While Red does not collect evidence, he encounters engrams at specific points, which are used to reconstruct memories.
Plot
Cole Black finds himself outside an abandoned building with only one thing on his mind: "save the girl." After infiltrating the building and killing the armed men he finds there, he discovers the girl tied to a chair with a bomb strapped to her chest. Black is unable to defuse the bomb before it explodes.
Black suddenly wakes up in an old, abandoned asylum, with a strange device strapped to his head. He is introduced via television screens to a shadowy figure identifying itself as Red, who tells him the device is known as the Pandora, or Savant, an experimental technology designed to record and play back human memory for analytical purposes. In this case, Red wants Black to explore the circumstances surrounding the girl's kidnapping, claiming that Black had something to do with it, although he cannot remember anything. While making his way through the asylum, Black encounters several fellow inmates, whose fates are ultimately determined by his actions. Many of them refer to Black as the "Puppet Master" and make other references to Alice in Wonderland.
Through the examination of several of his own memories and others provided by Red, Black begins to piece together what happened. He was hired by a man named Robert Ramsey to infiltrate ADS, a weapons contractor, and steal a prototype of their latest invention, the CornerGun. Ramsey then patented the CornerGun as his own creation, nearly bankrupting ADS and humiliating its CEO, Roger Howard. As a reward, Ramsey made Black the head of security for his own company.
Black manages to recall the name Jasper Prado, an Irish mercenary who was killed in suspicious circumstances around the same time as the kidnapping. Further investigation reveals that Jasper was hired by Rose Atkins, Robert Ramsey's research assistant, with whom he had an affair. Atkins, an ambitious, amoral woman, felt she was not getting the credit she deserved for the Pandora project and decided to betray Ramsey. She hired Prado and his men to kidnap Ramsey's daughter Grace and demand the Pandora device as ransom. Black frequently stumbles upon memories that depict Ramsey as a driven man whose obsession with memory—presumably due to his mother's succumbing to some form of mental illness—and the hope that his Pandora device will change the world causes him to neglect his wife Lenore and Grace. Based on these flashbacks and Red's angry response to them, Black deduces that Red is really Robert Ramsey, who confirms this and declares his intention to find out why Grace was taken.
Black remembers himself confronting Rose about the kidnapping and then throwing her out a window, killing her. When Ramsey questions his motives, Black cannot offer a definitive answer. Black's final memory is of himself being approached by Howard, who propositions him to steal the Pandora device as revenge for the CornerGun theft, in exchange for a hefty reward and a purging of Black's criminal record (which Ramsey is apparently using as leverage). Black flatly refuses; Ramsey, however, does not believe this either, and reveals that Black is not in an asylum at all, but incapacitated in a life support chamber. The "world" dissolves around Black as he screams for mercy.
