Desmond Miles, an ordinary bartender, is abducted by a mysterious organization in 2012. The captors attach him to the "Animus" device, allowing him to access the memories of his ancestor, Altaïr, an assassin from the Third Crusade era. The reconstruction of Altaïr's memories is crucial to the captors.
As Desmond experiences Altaïr's memories through the Animus, he relives events in 1191 Holy Land, where Al Mualim orders Altaïr to kill nine historical figures deemed responsible for war and corruption. The player controls both Altaïr and Desmond during different sessions of the Animus.
The game features innovative freedom of choice and movement mechanics. Players can use architectural details like ledges and carvings to navigate walls, leading to intense chase scenes if pursued by guards.
Players can gather information about targets using eavesdropping, interrogation, and pickpocketing, then choose methods for completing objectives, such as stealthy rooftop approaches or frontal assaults. The outcome depends on the player's choices, affecting interactions with civilians and knights.
Altaïr wields a small arsenal of swords and throwing knives, and can learn new combat skills to aid in his missions. His effectiveness depends on the player's strategy and tactics.