Desmond Miles is a bartender who is kidnapped by a powerful corporation called Abstergo. While captive, Desmond is informed that he can use a machine called the Animus created by Abstergo to relive his ancestors' memories. He consents to use the machine and relives the memories of his ancestor Altair ibn-La'Ahad.

Altair, Desmond's ancestor, is a disgraced assassin because he broke the three tenets of the Assassin's Creed:

1.Stay your blade from the flesh of an innocent.

2.Hide in plain sight.

3.Never compromise the brotherhood.




In order to regain his former status, Altair must hunt down nine key targets. Altair reluctantly agrees and he slowly hunts down the men. He realizes they all have something in common. After assassinating all nine targets and discovering that his ninth target was in fact a trap, Altair confronts Robert, the leader of the Knights Templar, in front of King Richard I of England, leader of the Crusader army. Altair informs Richard that Robert is plotting to betray and kill him, which causes him to encourage a duel to the death between Altair and Robert. Richard then believes Altair. Upon dying, Robert confesses to Altair that there were not nine, but ten Templars conspiring to take the Piece of Eden, the tenth being the head of the Assassins, Al Mualim, who wanted to keep the Piece of Eden for himself and therefore sent out Altair to kill the others. Back at Masyaf, the assassin's fortress, Al Mualim had taken over the minds of the people inside. Altair manages to defeat and kill him in a duel. The memory Abstergo was searching for, which located the Piece of Eden, reveals it is only just one of 28 that are scattered across the world.



Back in 2012, Abstergo sends a team to recover as many Pieces as possible. Deciding they do not need him anymore, they give orders to kill Desmond. Lucy Stillman, an Abstergo employee who revealed to Desmond that she is an undercover Assassin, convinces them not to kill Desmond until they recover the Pieces, buying him time.



Desmond is left alone in the Animus room, and by the use of Altaar's Eagle Vision inherited from the "Bleeding Effect" caused by the Animus, he sees many symbols drawn in blood on the wall in his room and on the floors of the lab. These symbols contain references to the end of the world in many different languages and in the left side of his room, the player sees "I Entered the Animus and never returned".