Watch_Dogs follows a man that can control the electrical grid of the city of Chicago. He has mobile hacking technology, which allows him to match people's real life identities with their online presences; as well as this, he can shut down mobile phone reception and listen in on phone calls.
Players can also take on the roles of the titular watch dogs, protectors of Aiden Pearce, as he is chased by police. Multiple players exist in the city at one time, although when this multiplayer triggers or what other players would be doing when Aiden is not in trouble is unclear.
In 2003, a disgruntled employee at a New York power plant secretly planted a virus in his company's computer system, which led to a massive blackout that disrupted power and communications to the entire northeastern US, resulting in hundreds of injuries and even a small number of deaths. 10 years later, a system called "CtOS" (Centralized Operating System) has simplified these connections, connecting entire cities in one gigantic network. The computers that run the CtOS are corporations interested in collecting the massive "data shadows" of every person, for uses from selling products to influencing worldviews.
While the CtOS makes such connections easier, it is only a matter of time before someone figures out a way to hack into this network and use or abuse it to suit their own purposes, and when that happens, the damage could be even more catastrophic.
Enter the "Watch_Dogs", a group of vigilantes who have found a way to exploit this network to deal with criminals where normal law enforcement can't or won't do the job.
From gameplay demos thus far, we can discern that in this open-world environment Aiden Pierce has multiple smart-phone abilities that affect the environment of the city. He can hack into any device to receive information, gain abilities, or see through any camera. He can stop trains, raise security barriers, and black out the entire city. He can slow down time to shoot. He can direct companions through stealth sequences. And in all this he can call in the help of mobile players if he gets in a bind.
When the game was announced at the Ubisoft E3 2012 conference, a lengthy gameplay demonstration was shown. In this demonstration, the protagonist is shown controlling the electricity grid, which allows him to manipulate street lights. This power was used to create a pile-up to entrap a digital artist named Joseph Demarco. Another section of the demo shows him trying to gain access to a building. In order to distract the guard at the door he shuts down communications in that area of the city at which point calls are cut off for NPC's. This causes enough confusion that the guard effectively abandons his post and allows the player into the building.
At Sony's Playstation Meeting where they revealed the PS4, Ubisoft played through a brief open-world section of the game. "There is no mission," they said, as Aiden walked the streets of Chicago. Most NPCs he encountered had dossiers projected for Aiden to see, revealing a homeless man to be an Iraq War veteran and a suited man to be a Tobacco Executive and Pro Life activist. Using a radial menu Aiden hacks the suited man's bank account and takes out of a nearby ATM.
Aiden cannot control everything. A security camera run by another entity was marked in red and secure. He can, however, using background histories and probabilities, detect the likelihood of events. Because a woman who had recently won a child custody battle was walking near an alley where her former husband was waiting, a crime probability bar began filling up as she walked toward the ambush. Aiden had the choice to help her and he did, interceding when the man put her at knife point. There is then an optional chase sequence where cops are alerted due to gun fire. In an alley, Aiden takes control of an electrical panel, overheats it, and takes down the man. To escape the cops, he stops an El-Line train and hops aboard.
As well as Aiden's ability to use technology to hack into systems and manipulate electrical devices, he can also employ many other skills to accomplish his goals. These include sneaking around environments and hiding behind cover, parkour-like actions to quickly traverse the city, and the ability to slow down time.
At the Sony Press Conference at E3 2013, another gameplay demo showed Pierce navigating an alerted city in order to save his colleague T-Bone from discovery. Pierce drove his car stealthily through the city streets, using the mini-map and environmental awareness to avoid patrolling police vehicles. When spotted by a helicopter search light, however, he drove to a hidden parking garage on Chicago's Lower Wacker Drive, losing the cops and taking the back entrance into a humming coffee shop. However, due to his raised alert status, the TV in the shop projected his image and Pierce had to smash the cell phone of a patron calling 911. This sequence demonstrated the real-world impact of a raised alert, where even after breaking line of sight the player must be cautious as any TV could begin the chase anew.
Pierce reaches his destination of a nondescript building where he hacks into the security cameras, then into a laptop camera to locate T-Bone, who has found a piece of computer hardware that impresses him. Pierce appears to need line of sight in order to hack and affect the world, but he can gain it through any camera in the world. In this case, he sees a screen array that he can turn on to distract the guard and give T-Bone an exit. Pierce then takes on a squad-command role as he directs T-Bone across a courtyard patrolled by guards, with T-Bone following the player's commands. Pierce can turn the lights on the parked cars on to distract guards as well.
However, they are discovered as they escape and Pierce kills a private security man before being sighted by a police helicopter.
The "second-screen" functionality was then revealed further as Pierce called for a random mobile user's help. This user, on a tablet, is playing at the CtOS system itself. The user shutdown the helicopter's systems for a second, momentarily disabling it but not destroying it, giving Pierce the time to get away.
He runs into some cops, though, and must employ more gadgetry. On his smart-phone select screen of abilities, he chooses total blackout, which kills every light in the city. He then uses his unexplained slow motion power to aim and shoot out the cops knee caps before escaping on a boat down the Chicago river.