The story DLC is dubbed The Lost Archive. It focuses on Subject 16, includes Lucy, and details 16's entry into the Assassins, and his capture by Abstergo. It was released on February 28, 2012. Following Subject 16's apparent death by drowning in the Tiber river, 16 is recruited by the Assassins to infiltrate Abstergo and steal information relating to the prototype Animus. After learning of the importance of the Animus to Abstergo, the Assassins arrange for 16 to be captured, later to be freed by Lucy, who is revealed to be a Templar triple agent. It is heavily implied she was enraged at her apparent abandonment at Abstergo by the Assassin order. 16 also inadvertently targets Desmond as another target by revealing Ezio Auditore as the Assassin Mentor. Lucy, realizing 16 has learned of her allegiance, allows 16 to be driven insane by the Animus and is planning to stage escape with Desmond so that he can lead the Templars to another Apple of Eden. After completing the sequence, 16's conscious is caught in "The Loop" a cycle of memories buried within Desmond. After collecting disparate fragments, a backdoor is revealed, allowing 16's consciousness to send a parting letter to his father, with whom he had a constantly strained relationship due to his mother's abandonment
Source: Wikipedia, "Assassin's Creed: Revelations", available under the CC-BY-SA License.
During his time in the Black Room, Desmond Miles relived the memories passed to him by a virtual construct of Clay Kaczmarek, his predecessor in the Animus Project. The memories related to Clay becoming a member of the Assassins, being assigned to infiltrate Abstergo Industries' Animus Project laboratory in Italy and discovering what Warren Vidic hoped to achieve by exploring the genetic memories of various Assassin figures.
Lucy Stillman, an Assassin herself, had been placed within the facility to rescue Clay when his mission was complete. In his memories, Clay recalled his father's disregard for his career goals, demanding that Clay should instead become an engineer to provide for his family. Clay believed that his father's focus on wealth pushed his mother away from the family.
Clay eventually realized that Vidic was using these genetic memories to discover the locations of the Pieces of Eden in the modern era. In particular, Vidic focused on locating an Apple of Eden, which was meant to be launched aboard a satellite to control the minds of humanity across the Earth. Clay also discovered Lucy discussing her true loyalty to the Templars with Vidic, and revealed her actual motives, which were to remove Desmond from the Abstergo facility and transport him to a more comfortable environment, where she would use the Assassins' resources to allow Desmond to discover the location of the Apple and recover it for the Templars.
Lucy, having sworn to protect Clay, wiped the security feed showing that he had seen their discussion, but did not let him leave the facility, to prevent her true allegiances from being discovered. The memories continued in a loop until Desmond collected the scattered memory fragments, which revealed Clay emailing his father and telling him not to worry about his disappearance, because he had found a greater purpose to serve, foreshadowing his suicide.
This DLC pack is available for purchase separately and bundled with the Ottoman and Gold Edition.
Source: Wikipedia, "https://en.wikipedia.org/wiki/Assassin%27s_Creed%3A_Revelati... Assassin's Creed: Revelations]", available under the CC-BY-SA License.