Dr. Martin Harris (Liam Neeson) and his wife Liz (January Jones) arrive in Berlin for a biotechnology summit. Upon arriving at their hotel, Martin realizes he forgot his briefcase 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 after narrowly avoiding a refrigerator that falls off a delivery truck in front of them. Martin is knocked unconscious upon impact, but Gina saves him from drowning before fleeing from the scene. On Thanksgiving day, he comes to 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 U.S. Martin then heads to the office of Prof. Bressler (Sebastian Koch), whom he is scheduled to meet that day. However, he sees the impostor (known as "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, where he undergoes an MRI to see if the accident had caused any head trauma. An assassin sent to target him kills Dr. Farge (Karl Markovics) and Nurse Gretchen Erfurt (Eva Löbau), but Martin manages to escape assassination. He then seeks help from private investigator and former Stasi agent Ernst Jürgen (Bruno Ganz). Martin's only clues are his notebook and Gina, who works at a diner after being fired from the taxicab company. 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 (Mido Hamada), who is funding a secret project headed by Prof. Bressler. Prince Shada has survived numerous assassination attempts from Islamic militants, and he suspects that Martin's identity theft may be another attempt at taking the Prince's life. Jürgen meets up with Cole, but when he realizes that Cole is not who he says he is, he commits suicide by drinking cyanide-laced tea, keeping the whole truth away from Cole. Meanwhile, the assassins attempt to eliminate Martin and Gina, but the couple escape after a fight at her apartment and a lengthy car chase. Martin looks at the last page of his notebook and sees a set of numbers written by his wife. 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 his only hope is his briefcase at the airport. After retrieving his briefcase, Martin parts ways with Gina, but is tazed and nabbed by Cole and his henchman Jones. Seeing the abduction, Gina steals a taxicab and chases after Cole's van. Martin wakes up at a car park and discovers the truth of his identity through Cole. It is revealed that Martin Harris is a name he made up, and that he, Liz and Martin B are part of a terrorist organization sent to target the summit. None of his memories are real, but he started to believe he was Martin Harris after the accident. Gina then rushes in and stops Cole and his henchman from killing Martin through a drug overdose, sending the van crashing outside the car park. Martin discovers a hidden compartment in his briefcase and finds two Canadian passports, realizing he and Liz were in Berlin three months prior to plant a bomb in the suite that is to be occupied by Prince Shada. The duo head for the hotel to stop the assassination. Meanwhile, Liz steals Prof. Bressler's data by secretly attaching a USB flash drive and operating it remotely, using the secret codes on the notebook she and Martin carry. Martin and Gina are immediately nabbed by security, but he convinces the security head of his presence in the hotel three months back. He then realizes that Prince Shada is not the target, but Prof. Bressler - who has discovered a genetically modified breed of corn that is capable of surviving any climate. After being convinced there is a bomb, security evacuates the hotel. Seeing that their assassination attempt has been foiled, Liz tries to disarm the bomb, but is too late, as she is killed while a section of the hotel is destroyed. Martin B, the last remaining terrorist, attempts to kill Prof. Bressler, but Martin stops and kills him. Soon afterward, Martin and Gina, who now carry Canadian IDs (that have the same last name, implying that they are married) leave on a train, now free from their troubled lives.
Many German actors were cast for the movie. Rainer Bock had previously starred in Inglourious Basterds (which also starred Diane Kruger) and The White Ribbon. Other cast includes Adnan Maral as Turkish taxi driver and Petra Schmidt-Schaller as immigration officer.Principal photography took place in early 2010 in Berlin, Germany, and in the Studio Babelsberg film studios. The bridge the taxi plunges from is the Oberbaumbrücke. The Friedrichstraße was blocked for several nights for the shooting of a car chase. Some of the shooting was done in the Hotel Adlon. Locations include the Neue Nationalgalerie, Berlin Central Station, Berlin Friedrichstraße station, Pariser Platz, Museum Island, the Oranienburger Straße in Berlin, and the Leipzig/Halle Airport. According to Andrew Rona, the budget was $40 million. Dark Castle Entertainment contributed $30 million. German public film funds supported the production with €4,65 million (more than $6 million).Unknown was screened out of competition at the 61st Berlin International Film Festival. The film has received mixed to positive reviews, scoring 56% on Rotten Tomatoes, with an average score of 5.8/10, with the general consensus being "Liam Neeson elevates the proceedings considerably, but Unknown is ultimately too derivative -- and implausible -- to take advantage of its intriguing premise". Considerable praise has been placed on Neeson in the lead role and the unique premise of the film, and has comparisons with Neeson's 2008 film Taken. Richard Roeper gave the film a B+, reflecting "At times, Unknown stretches plausibility to the near breaking point, but it's so well paced and the performances are so strong and most of the questions are ultimately answered. This is a very solid thriller."