Assassin's Creed Official Group

The overall story within Assassin's Creed is set in September 2012, where barkeeper Desmond Miles (the character controlled by the player) has been kidnapped by an unknown person prior to the start of the game, and is brought to Abstergo Industries, where researchers are working on the Animus project. The Animus is able to pull out memories from the DNA of the user, 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 Al-taa-ir in Arabic "The Flying One") a member of the Assassins during the Third Crusade of the Holy Land; the researchers express interest in understanding more of how the Assassins worked. Though initially Desmond's sub-conscious 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 AD 1191: Richard the Lionheart has just recaptured the port city of Acre from the Conquer of Muslims. 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 Muslim forces are massing in the ruins of 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 failing to assassinate the Grand Master of The Templar Knights, Robert de Sable, and recover the order's treasure. Due to this, Altaïr is demoted to Uninitiated (the lowest rank in the Assassin Order). But Al Mualim, leader of the Assassins, offers him an opportunity to redeem himself. 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, allowing Mualim to usher in an age of peace.

Assassin's Creed Info.[en.wikipedia.org]
Assassin's Creed II Info.[en.wikipedia.org]
Assassin's Creed Website[assassinscreed.us.ubi.com]