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Dr. Martin Harris (Liam Neeson) and his wife Liz (January Jones) arrive in Berlin for a biotechnology summit. At the hotel, Martin realizes he left his briefcase at the airport and takes a taxi to retrieve it. On the way, the taxi is involved in an accident and crashes into the Spree, knocking Martin unconscious. The driver, Gina (Diane Kruger), rescues him and flees the scene, as she is an illegal immigrant from Bosnia. Martin regains consciousness at a hospital after having been in a coma for four days.
When Martin returns to the hotel, he discovers Liz with another man, and she claims to not know him. Martin phones a colleague in the States, Prof. Rodney Cole (Frank Langella) but reaches only his voice mail. Aboard a train, Martin writes down his schedule for the next day by memory. Martin visits the office of Prof. Leo Bressler (Sebastian Koch), whom he is scheduled to meet, but sees the impostor, "Martin B" (Aidan Quinn), already there. As Martin attempts to prove his identity, Martin B shows him his ID and family photo, both of which have the impostor's face. Overwhelmed by the identity crisis, Martin falls unconscious, then finds himself back at the hospital. Smith (Olivier SchneiderÂ (fr)), an assassin sent to target Martin, kills a nurse, Gretchen Erfurt (Eva LÃ¶bau), but Martin escapes.
He seeks help from Erfurt's friend, private investigator and former Stasi agent Ernst JÃ¼rgen (Bruno Ganz). Martin's only clues are his father's book on Botany and Gina, who since the crash now works at a diner. While Martin persuades Gina to help him, JÃ¼rgen digs up information related to Martin and the biotechnology summit. He discovers the summit will be attended by Prince Shada (Mido Hamada) of Saudi Arabia, who is funding a secret project headed by Bressler. Prince Shada has survived numerous assassination attempts by extremists in his own country, and JÃ¼rgen suspects that Martin's identity theft may be part of another attempt.
At Gina's apartment, Smith and another terrorist, Jones (Stipe Erceg), attack; the couple escapes after Gina kills Smith. In his book, Martin finds that Liz has written a series of numbers that correspond to words found specific pages. Using his schedule, Martin confronts Liz alone; she tells him he left his briefcase at the airport. Meanwhile, JÃ¼rgen receives Cole at his office and deduces that Cole is a former mercenary and potent killer. Realizing Cole is there to kill him and he has no way of escaping, JÃ¼rgen commits suicide by ingesting cyanide to protect Martin.
After retrieving his briefcase, Martin parts ways with Gina. When she sees him kidnapped by Cole and Jones, she steals a taxicab and chases them. When Martin wakes, Cole says Martin Harris is a cover name in an assassin team including Liz to target the summit. Liz notified Cole that Martin injured his head during the car crash altering his memory and causing him to believe his false Martin Harris persona was his own identity. Cole explains, that's why Martin B was activated as his replacement. Gina runs over Jones before he can kill Martin, then rams Cole's van, killing him. After Martin finds a hidden compartment in his briefcase containing two Canadian passports, he remembers that he and Liz were in Berlin three months prior to plant a bomb in the suite to be occupied by Prince Shada.
Now aware of his own role in the assassination plot, Martin seeks to redeem himself by thwarting it. He heads to the hotel with Gina. Security immediately arrests them, but Martin convinces them of his presence in the hotel three months earlier. Liz uses her own copy of the book's secret codes to remotely access Bressler's laptop and steal the data. After being convinced of the bomb's presence, security evacuates the hotel.
Martin suddenly realizes that Prince Shada is not the target, but Bressler, who has developed a genetically modified breed of corn capable of surviving harsh climates. With Bressler's death and the theft of his research, billions of dollars would fall into the wrong hands. Seeing their assassination attempt has been foiled, Liz dies attempting to disarm the bomb. Martin kills Martin B, the last remaining terrorist, before the latter can murder Bressler. Bressler announces that he is giving his project to the world for free, while Martin and Gina board a train with new passports and identities.

Question: Who is the private investigator?
Answer: Ernst Jurgen (Bruno Ganz)