Excerpt from [wikipedia]:
“Revelations features all three of the series’ major characters so far: Desmond Miles, Altaïr ibn La-Ahad and Ezio Auditore da Firenze. The main part of the story takes Ezio to Constantinople in 1511 AD, consisting of four districts: Constantin, Beyazid, Imperial, and Galata; Cappadocia during the rise of the Ottoman Empire, part of which is a completely underground city populated solely by Templars; and Masyaf, where the old Assassins’ stronghold is located (featured in the first Assassin’s Creedgame), to which Ezio travels at the beginning of the game. He discovers Altaïr has sealed within the fortress an ancient artifact that is said to be a powerful weapon which could end the Templar–Assassin War forever, and had the keys hidden in Constantinople. Ezio uses these relics of the “First Civilization” that hold memories of Altaïr to relive Altaïr’s experiences, during which players will control Altaïr.
The game also continues Desmond’s story in the present day, following the events of Brotherhood, where he is trapped in the Animus, in a comatose state, in which he has found a safe mode known as “the Black Room”. Here, Desmond must see every key memory that links him with Altaïr and Ezio to reintegrate his splintered subconscious and awaken from his coma. While in game, Ezio meets a variety of historical characters, including: Manuel Palaiologos, an heir to the now-lost Byzantine Empire; Byzantine Templars lurking in the shadows of Constantinople; and Prince Suleiman, a man who will one day become one of the Ottoman Empire’s greatest Sultans; and his uncle, Prince Ahmet.”