The player character is a modern-day man named Desmond Miles who, through a machine called the Animus, relives the genetic memories of his ancestor, Altaïr Ibn-La'Ahad.

While most of the game takes place within a simulation based on Altaïr's memories, the player will occasionally be forced out of the Animus to play as Desmond in the modern day.

Here, they are restricted to exploring a small laboratory facility, as Desmond has been kidnapped by Abstergo Industries, a shady corporation looking for specific information within Altaïr's memories that will further their enigmatic goals.

The sequel continues the modern-day narrative following Desmond but introduces a new storyline set during the Italian Renaissance in the late 15th century and a new protagonist, Ezio Auditore da Firenze.

Assassin's Creed is an action-adventure game,[5] set in an open-world environment,[6] which is played from a third-person view in which the player primarily assumes the role of Altaïr, as experienced through protagonist Desmond Miles.

Low-profile actions allow Altaïr to blend into nearby crowds, pass by other citizens, or perform other non-threatening tasks that can be used to hide and reduce the alertness level.

To reduce the alert level, the player must control Altaïr to break the guards' line of sight and then find a hiding space, such as a haystack or rooftop garden, or blend in with the citizens sitting on benches or wandering scholars.

When the synchronization bar is full, the player has the additional option to use Eagle Vision, which allows the computer-rendered memory to highlight all visible characters in colours corresponding to whether they are allies (blue), foes (red), neutral (white), or the target of their assassination (gold).

[8] In 2012, bartender Desmond Miles (Nolan North) is kidnapped by agents of Abstergo Industries, the world's largest pharmaceutical conglomerate, and is taken to their headquarters in Rome, Italy.

Under the supervision of Dr. Warren Vidic (Philip Proctor) and his assistant Lucy Stillman (Kristen Bell), Desmond is forced to enter a machine called the Animus, which can translate his ancestors' genetic memories into a simulated reality.

In 1191, Altaïr and two fellow Assassins—brothers Malik (Haaz Sleiman) and Kadar Al-Sayf (Jake Eberle)[10]— are sent to Solomon's Temple to retrieve an artifact known as the Apple of Eden from the Brotherhood's sworn enemies, the Knights Templar.

Although Altaïr later partially redeems himself by fighting off a Templar attack on the Assassin home base of Masyaf, his mentor and superior, Al Mualim (Peter Renaday), demotes and orders him to assassinate nine individuals in order to regain his previous position and honour:[11] As Altaïr eliminates each target, he discovers all nine are Templars who had conspired to retrieve the Apple, revealed to be a relic of a long-forgotten civilization said to possess god-like powers.

After Altaïr mortally wounds him, Robert identifies Al Mualim as the final conspirator, revealing that the latter has betrayed both the Assassins and Templars to acquire the Apple.

Altaïr returns to Masyaf, where Al Mualim has used the Apple to enthrall the population, as part of his plan to end the Crusade and all conflict in the world by imposing order by force.

With the help of Malik and several Assassins brought for backup, Altaïr storms the citadel and confronts Al Mualim in the gardens, resisting the Apple's powers and killing his mentor.

As Microsoft and Sony had not yet revealed their next consoles, the game's initial development from January 2004 was aimed as a PlayStation 2 title, following the same linear approach that The Sands of Time had taken.

[12] As more information came in about the capabilities of the next-generation consoles by September 2004, Désilets' team considered expanding the Prince of Persia acrobatic gameplay into an open world that would be feasible on the newer systems.

The team wanted Altaïr's parkour moves to look believable but sacrificed realism for gameplay value, allowing the player to make maneuvers otherwise seemingly impossible in real-life.

Historically, these cities had such landmark towers, and inspired by those, the developers incorporated them into the map, making these points of interest and challenges for players to drive them to climb them.

[13] Désilets had come from an acting background, and one element he had incorporated into the game was to make the player-character feel more like they were controlling parts of a puppet so that the character would appear more human.

[13][14] Following the 2006 E3 presentation and the name change to Assassin's Creed, the Ubisoft Montreal team grew to support the last year of the game's development, with up to 150 persons by the end of the process.

[14] Added team members included those that had just finished up production on Prince of Persia: The Two Thrones, as well as former staff that had recently been let go from Gameloft, another publisher owned by Ubisoft's co-founder Michel Guillemot.

About five days before they were to have sent the final software version for mass production, they were contacted by their CEO, who said his son had played the game and found it boring and gave them a missive to add side quests.

The team spent the next five days hastily rushing to add the collecting side quests to give the game more depth and ensure these were bug-free to make their mass production deadline.

[25][26] Al Mualim's character is roughly based on Rashid ad-Din Sinan, who in 1191 was the leader of the Syrian branch of the Hashshashin in the Nizari Ismaili state and was nicknamed "The Old Man of the Mountain".

[27] On October 22, 2007, an IGN Australia interview with Patrice Desilets mentioned that the lead character's climbing and running were done by "Alex and Richard – the same guys from Prince of Persia".

Ubisoft purposely inserted a computer bug into the pre-release version to unpredictably crash the game and prevent completion as a security measure; however, players were able to use extra content available on the Internet to bypass it.

[48] On The Hotlist on ESPNews, ESPN's Aaron Boulding called the game's concept of social stealth "fairly original" and added: "Visually, the developers nailed it.

[51] Kevin VanOrd of GameSpot gave it a 9 out of 10, stating that "the greatest joy comes from the smallest details, and for every nerve-racking battle, there's a quiet moment that cuts to the game's heart and soul.

Conversely, he complimented the combat animations and the climbing mechanic, and admired how accurately Ubisoft depicted the major cities of Jerusalem, Acre, and Damascus to their real-life counterparts.