Set within a fictionalised representation of a futuristic, dystopian London, the game's story follows the hacker syndicate DedSec as they seek to clear their names after being framed for a series of terrorist bombings.

While searching for the true culprits, DedSec also attempt to liberate London's citizens from the control of Albion, an oppressive private military company that has turned the city into a surveillance state following the bombings.

Ubisoft supported Legion after its launch, releasing a number of updates for both the single-player and multiplayer modes that added new missions, game modes, and playable characters; a crossover event with the Assassin's Creed franchise; and a paid story expansion, Bloodline, which continues the storylines of Aiden Pearce, the protagonist of the first Watch Dogs game, and Wrench, a major supporting character from Watch Dogs 2.

Watch Dogs: Legion is an action-adventure game played from a third-person perspective, and taking place within an open world setting based upon London, which can be explored either on foot─utilizing parkour moves─vehicles, or fast-travelling via the city's Underground stations.

Players can be pursued by enemies when escaping, including hostile drones, but can lose them by utilizing hack-able environmental objects (e.g., vents) and avoiding line of sight with pursuers.

Some recruits may not join if either they favour those that oppose them (such as a hostile faction), if the player has a character in their roster whom they hate, or if DedSec did something to harm another NPC they have good relations with.

[6][7] Examples of such a mission include sneaking into a government building to find a missing person, recovering confiscated or stolen equipment or simply helping the potential recruit determine why they are experiencing invasive surveillance.

The online component of the game, introduced in March 2021, allows for four-player cooperative gameplay, which aimed to share progression between the single-player and multiplayer modes.

[9] Like the single-player mode, players can freely explore London and recruit new operatives to their team; however, rather than completing short missions for each character, this is done by spending "Influence" (an in-game form of currency).

Influence is also used to unlock gadgets and character upgrades, and can be earned from completing missions and daily/weekly objectives, bought with real-life money from the in-game store, or found across the map, along with masks and experience points (the locations of collectibles change weekly).

Since the events of Watch Dogs 2, technology has undergone a rapid acceleration as a result of the proliferation of automation and artificial intelligence (AI), which has effectively improved Britain's economy at the cost of many blue and white collar jobs.

AR and VR systems are commonplace across the city, accompanied by an increase in drones and electric cars, an established 6G mobile network, and the introduction of self-driving cars much of which is enhanced through the use of ctOS (central Operating System)─the centralized computer network developed by technology company Blume, featured in Watch Dogs and Watch Dogs 2.

The London branch of DedSec, led by Sabine Brandt and her newly crafted AI, Bagley, detect armed intruders planting explosives in the Houses of Parliament.

Although Dalton manages to prevent Parliament's destruction, he is gunned down by drones commanded by Zero Day's leader, who detonates additional explosives around London and orders an attack on DedSec's main hideout, forcing Sabine to shut down Bagley and go into hiding.

In the wake of the bombings, the British government contract Albion with restoring order to London and hunting down DedSec, who are held responsible for the chaos, effectively causing social and political unrest.

Months later, Albion enforces the law without political oversight, transforming London into a surveillance state with the aid of its ctOS network and SIRS, a conglomerate of Britain's intelligence agencies.

As a result, citizens have their personal liberties severely restricted and their lives constantly monitored,[6] while those who question Albion's methods are either convicted or deported to Continental Europe.

DedSec slowly rebuild their strength as they find more recruits who, under Sabine's co-ordination and with Bagley's help, liberate London's boroughs by encouraging citizens to rise up in defiance of their oppressors.

DedSec next sabotage Cass's drone project and expose his crimes to the public, prompting him to take refuge at Albion's headquarters in the Tower of London.

While the British government reviews its contract with Albion and the MPS begins work to resume operations, DedSec finally clear their names and are praised for exposing considerable crimes and corruption across London.

Aiden's assignment is to infiltrate Broca Tech and acquire photographic evidence of a new robot design project headed by Thomas Rempart, as well as retrieve a device called the "BrocaBridge".

They also discover that Darcy returned to London to rescue her brother Lucas, who was captured while trying to free the city from the Templars' control like their ancestors had done nearly two centuries prior.

After tracking down Darcy, DedSec agree to help her rescue Lucas, who is being tortured by Graham to learn the location of a hidden Assassin Tomb in London.

[13] Upon its reveal at E3 2019, many outlets described the futuristic London setting as post-Brexit, what could potentially happen following the expected departure of the United Kingdom from the European Union.

[15][31] The game's multiplayer component was initially scheduled for a 3 December release, but was pushed back to early 2021 due to numerous game-breaking glitches and bugs that needed to be fixed.

Season Pass exclusive content included a new mission for the single-player mode, "Swipe Right", and a new playable character, Mina Sidhu, a former test subject who has gained mind control powers.

[40] A spin-off comic book series was released monthly beginning in November 2021 by Behemoth in the United States and in two collected editions in France by Glénat.

[41][42] A prequel novel to the Bloodline expansion, titled Stars and Stripes, which follows Aiden Pearce and bridges the gap between the events of Watch Dogs and Legion, was published by Aconyte Books in April 2022.

The lack of uniqueness in the NPCs' accents was also criticised, with Aitken adding that players would mostly find it useful to go primarily for those with hacking skills, due to how much of the game requires these.

She concluded that "unlike the glossy, beautiful, but samey open-worlds that have dominated the genre in the past few years, [Legion] is ambitious, imperfect and unashamedly weird" and gave it 4/5 stars.