The games use the concept of active versus passive moves

The games use the concept of "active" versus "passive" moves, with "active" moves, such as running, climbing the sides of buildings, or jumping between rooftops, more likely to alert the attention of nearby guards. When the guards become alerted, the player must either fight them or break their line of sight and locate a hiding place, such as a haystack or a well, and wait until the guards' alert is reduced. The combat system allows for a number of unique weapons, armor Blazblue Costume
, and moves, including the use of a hidden blade set in a bracer on the Assassin's arm, and which also can be used to quietly assassinate targets.

Desmond is captured by Abstergo and forced to use a machine called the Animus to explore Altaïr ibn-La'Ahad's memories during the time of the Third Crusade. Desmond begins to witness events after Altaïr breaks all three tenets of the Assassin Brotherhood while attempting to stop Robert de Sablé from taking a Piece of Eden. Al Mualim, the Brotherhood's leader, demotes Altaïr to Novice and tasks him with assassinating the nine Knights Templars, including de Sablé, to regain his former status. Altaïr's quest eventually leads him to face de Sablé in the presence of King Richard I of England warning the King of de Sablé's plot to kill him. Altaïr defeats de Sablé, but with his last words Cosplay Australia
, de Sablé reveals that there were ten Templars, the last being Al Mualim, who now holds the Piece of Eden. Altaïr returns to face Al Mualim, struggling to fight through the illusions created by the Piece, but eventually kills Al Mualim. When Altaïr recovers the piece, he—and those watching the Animus—witness a holographic map showing locations across the globe where other artifacts are located. Desmond is pulled from the Animus, and was going to be killed, but his life is spared by Lucy Stillman, an Assassin working as a mole within Abstergo, as she claims they might need to further examine his memories later. He comes to learn that a former test subject had left messages only Desmond can see, foretelling of the end of the world in 2012.

The first Assassin's Creed introduced elements that would remain cornerstones of the rest of the series. The game attempts to create historical versions of Masyaf (the Brotherhood's location), Jerusalem, Acre and Damascus, and incorporates a number of documented historical figures into the story. The virtualized Animus system, free running, climbing, stealth, and parkour elements were all present in this title, as well as the initial formulation of the combat system. The player would have to complete a number of side quests before they would be able to obtain an assassin quest from a local Brotherhood guidemaster in each city, though future games would abandon this prerequisite.

Lucy returns and breaks Desmond out of Abstergo, taking him to an Assassin safehouse and introduced to Shaun, a historian, and Rebecca, their technical support. Using an improved version of the Animus, Desmond takes witness to Ezio Auditore da Firenze, a young nobleman from the late 15th Century in Florence, shortly before the execution of Ezio's father and brothers by order of a corrupt official working for the Templars. He and his mother and sister take to hiding at the Monteriggioni villa owned by his uncle Mario, who helps to train Ezio in the ways of an Assassin. Ezio and Mario discover that Rodrigo Borgia is the figurehead of a number of Templars all who were accomplices in the execution of his family, and with help of allies like Leonardo da Vinci, methodically assassinate the lower-ranked Templars, while learning that Borgia has acquired a Piece of Eden called the Apple. Eventually, Ezio corners Borgia, who hands over the Piece and flees; Ezio is inducted into the ranks of the Brotherhood and believed to be a prophet based on their codex. A decade later, the Brotherhood learns that Borgia has since become Pope Alexander VI, using the Church's influence to strengthen the Templars. Ezio invades Vatican City and confronts Borgia, discovering that the Papal staff is also a Piece of Eden. Defeating Borgia but sparing his life, Ezio uses both the Apple and the Papal staff to reveal a chamber of ancient technology Castlevania Costume
. Inside, he is addressed by a hologram of a humanoid female that calls herself Minerva who speaks directly to Desmond through Ezio. Minerva explains how her society had pre-dated humanity's, but a great disaster nearly wiped them out, and warns that another event is due to occur soon, putting the fate of humanity in Desmond's hands. At this discovery, Desmond and his allies find the safehouse has been compromised by Abstergo and flee.