Assassin's Creed is a third-person stealth game in which the player assumes the role of Altaïr ibn La-Ahad (Arabic, "The Flying One, Son of None"), a member of the Hashshashin, The Assassin Brotherhood, which is not allied to either the Crusaders or Saracens during the third crusade. The brotherhood's primary enemy is the Knights Templar. Altaïr's objective in the game is to assassinate nine historical figures propagating the Crusades in the year 1191. According to Ubisoft developers, all of the main character's targets are historical figures who died or disappeared in 1191, although not necessarily by assassination.
The primary goal of the game is to carry out the assassinations ordered by the head of the Brotherhood, Al Mualim. To achieve this goal, the player must use stealth and a variety of intelligence gathering tactics to collect information on their target. These tactics include eavesdropping, interrogation, pickpocketing, and completing tasks for informers—fellow assassins who will give you information in exchange for assassinating targets or collecting flags. Additionally, the player may take part in any number of side objectives, including climbing tall towers to map out the city, and saving citizens who are being threatened or harassed by the city guards. There are also various side quests that do not advance the plot such as hunting down and killing Templars and flag collecting. - wikipedia
"The overall story within Assassin's Creed is set in September 2012 AD. Desmond Miles, a bartender, has been kidnapped by unknown people (Abstergo) prior to the start of the game, and is brought to a building belonging to Abstergo Industries, where researchers are working on a project involving a machine called the Animus. The Animus is able to extract memories from the DNA of the user, passed on from descendant to descendant, allowing the user to replay those memories as if he were there himself. Desmond was "chosen" for the project due to his relationship to his ancestor Altaïr Ibn La-Ahad (الطائر at-ta'er in Arabic meaning "Flying One" and his last name meaning "Son of the None"), a member of the Assassins Order during the Third Crusade of the Holy Land; the researchers express interest in understanding more of how the Assassins worked. Though initially Desmond's subconscious rejects the memories of Altaïr, he eventually learns to accept them and is able to proceed to follow Altaïr's actions through the Animus.
The memories of Altaïr that Desmond experiences start in the year 1191: Richard I of England has just recaptured the port city of Acre from the Saracens. With a base of operations established, the Crusaders prepare to march south. Their true target is Jerusalem, which they intend to recapture for Christianity. However the Saracen forces are massing in Arsuf, intending to ambush the Crusaders and prevent them from reaching Jerusalem. These war maneuvers have left the rest of the Holy Land wide open. While Richard and Saladin battle one another, the men left to govern in their stead have begun taking advantage of their newfound positions of power. Exploitation, manipulation, and provocation rule the day.
The first memory Desmond experiences is that of Altaïr, along with another assassin named Malik and his brother Kaddar, being ordered to retrieve a mysterious artifact from ruins of the Solomon's Temple in Jerusalem which is also being sought by The Knights Templar. As Altaïr and his companions make their way into Solomon's Temple, they discover that Grand Master of The Knights Templar, Robert IV de Sablé is also there. Altaïr attempts to kill Robert but, due to his arrogance and over-confidence, the ensuing attempt fails and Altaïr gets separated from his companions, leaving Robert and his men with Malik and Kaddar. Upon returning to the order's base of operations in Masyaf it is discovered that Malik survived the engagement; although losing an arm and his brother, he managed to retrieve the artifact. The Templars, however, followed him and attack the city. Altair manages to set off a defence mechanism, forcing the depleted Templar army to flee. Since Altaïr failed to kill de Sablé, and caused the Templars to attack them, the leader of the Assassins, Al Mualim, stabs him. He awakens and discovers that Al Mualim has, rather than killing him, decided to give him a second chance. He is demoted to Uninitiated, the lowest rank in the Assassin Order (where Altaïr had started off when he had joined as a child) and is told how to rise through the ranks of the Assassin Order to his original rank: Altaïr must venture out into the Holy Land and assassinate nine men said to be exacerbating and exploiting the hostilities created by the Third Crusade. In doing so, he will stabilize the region, helping the Order's goal of securing peace in the Holy Land.
Along the way, however, Altaïr discovers that his targets are bound by more than just a shared interest in personal gain, but are secretly members of the Knights Templar, with the goal to unite all mankind under a common cause. Desmond further learns by secretly reading emails on computers of the research personnel when left overnight between Animus sessions that Abstergo Industries is in fact run by the Knights Templar, the order having survived from the Middle Ages, and that the Templars are seeking the locations of several artifacts known as the "Pieces of Eden" that they can use to continue Templars' goals. Desmond learns that he was kidnapped so that they may learn, through Altaïr's memories, where the locations of the other Pieces of Eden may be buried throughout the world. He also finds that what remains of the Assassin's Brotherhood in modern days had tried and failed to recover him before that information could be learned. Altaïr's memories finally reveal that the assignment he undertook was a plot by the Templars to sow discord between the Assassins and the Crusaders and Muslims. In doing so, the Muslim leader Saladin and Richard The Lionheart would instead work together and fight the Assassins, and in the process, bringing peace for the Templars. Altaïr finally catches up with Robert de Sablé, his last assigned target, and defeats him. Robert de Sablé reveals in his dying words that Altaïr's master, Al Mualim, is also a member of the Templars, and he has now the sole power of the artifact, the "Piece of Eden" that is able to alter what men can see and can be used for mass hypnosis. Altaïr travels to face Al Mualim, who wields the "Piece of Eden", attempting to alter Altaïr's sight, but eventually falls to his blade. As Altaïr recovers the "Piece of Eden", the device activates one more time to reveal multiple locations across the globe in an holographic manner, including two locations on landmasses that no longer exist in 2012 AD." - wikipedia