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Dr. Martin Harris (Liam Neeson) and his wife Liz (January Jones) arrive in Berlin for a biotechnology summit. Upon arriving at their hotel, Hotel Adlon, Martin realizes his briefcase was left at the airport. He takes a taxicab driven by Gina (Diane Kruger), but on the way to the airport, the cab crashes off a bridge into the river. Martin is knocked unconscious upon impact, but Gina saves him from drowning before fleeing from the scene to avoid the police, since she is an illegal immigrant from Bosnia. On Thanksgiving day, he gains consciousness at the hospital after being in a coma for four days. Martin returns to his hotel, only to discover another man (Aidan Quinn) with his wife, who tells hotel security she does not know him. He attempts to contact his old colleague Prof. Rodney Cole (Frank Langella), who is unavailable due to Thanksgiving in the United States. Martin then heads to the office of Prof. Bressler, whom he is scheduled to meet that day. However, he sees the impostor, "Martin B", already in a meeting with Bressler. As Martin attempts to prove his identity, Martin B shows him his ID and family photo – both of which have Martin's name, only with his face replaced by that of the impostor. Disillusioned by the identity crisis, Martin falls unconscious and finds himself back at the hospital. An assassin sent to target him kills Nurse Gretchen Herfort, but Martin escapes. He then seeks help from Nurse Herfort's friend, private investigator and former Stasi agent Herr Jürgen (Bruno Ganz). Martin's only clues are his notebook and Gina, who works at a diner after being fired from the taxicab company since the incident. While Martin attempts to persuade Gina into helping him clear up his identity problem, Jürgen digs up information related to Martin and the biotechnology summit. He discovers that the summit is to be attended by Prince Shada of Saudi Arabia, who is funding a secret project headed by Bressler. Prince Shada has survived numerous assassination attempts from extremists in his own country, and Jürgen suspects that Martin's identity theft may be another attempt at taking the Prince's life. Meanwhile, the assassins attempt to eliminate Martin and Gina, but the couple escape after a fight at her apartment (in which the assassin who killed Gretchen is killed by Gina) and a lengthy car chase. Martin looks at his notebook and sees a set of numbers written by his spouse. The numbers correspond to words found on specific pages of the notebook, which appear as secret codes. Using the schedule written on the notebook, Martin confronts Liz, who tells him that he left his briefcase at the airport. Jürgen receives Cole at his office, but since he realizes that Cole is not who he says he is (surmising that Cole is a member of a secret for-hire assassination unit named Section 15), he commits suicide by drinking cyanide-laced coffee. He also has figured out who Martin really is (also a member of the same unit), because his last words are to ask Cole, "What if he [Harris] remembers everything? What then?"