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.