In a newspaper advertisement, taxi driver Tarek Fahd discovers an invitation to participate in an experiment. 4000 German marks are offered to the participants of the experiment, in which a prison situation is simulated. The experiment is led by Professor Klaus Thon and his assistant, Dr. Jutta Grimm.Tarek participates as a journalist while wearing a pair of glasses with a built-in mini-camera. Tarek has a car accident shortly before the experiment, after which he meets a woman called Dora. She spends the night with him and Tarek keeps thinking of her, shown in flashbacks. The 20 volunteers are pronounced guards and prisoners and are being observed by a team of scientists. The prisoners lose their civil rights and have to obey arbitrary rules, such as completely eating their meals. The guards are given nightsticks but are told not to use violence in any case. Each prisoners name is taken away and replaced by a number. Tarek (prisoner nr. 77) initially refuses to acknowledge the guards' superiority by drinking the milk of one of his co-prisoners because of that prisoner's lactose intolerance or by throwing his blanket out of his cell to provoke the guards. He befriends his cellmates, Steinhoff and Schütte. Psychological changes develop and the situation deteriorates. The circumstances seem to be escalating after a few days. It becomes clear that limits are not only being reached but being surpassed.The guards become excessively aware of their power and use the prisoners' fear to make them obedient. On both sides, one person is considered dominant. On the prisoners' side, this is Tarek, and aon the guards side, it is the quiet guard Berus, a sadist, whose motto during the experiment is: "Humiliation is the only way we can solve these troubles." From that moment, the guards start to use more and more violence against the prisoners. The scientists engage in a discussion whether or not to abort the experiment. Dr. Grimm suggests to put an end to the alarming situation, but Professor Thon refuses to stop the experiment until the violence has reached a maximum.Dora meanwhile returns to Tarek's apartment and discovers his participation contract for the experiment. She surprises him by showing up for a visit day. Tarek pleads to the friendly guard Walther Bosch to secretly bring her a message. Berus suspects him, however. He arrests Bosch and tells Dora that everything is alright. The situation becomes critical and more violence is involved. The prisoners are being abused and their self-esteem is drastically decreased. Most of the violence is against Tarek. First, the guards kidnap him from his cell late one night, order him to strip fully naked, shave his head bald and urinate on him. Later he is forced to clean the toilet with his own clothes. In the end, they lock him up in solitary confinement inside a "black box" resembling a safe. Schütte, reacting to this, is beaten severely, tied up, his mouth bound with duct tape and set on a chair. Later he suffocates to death due to his bloody nose drying up in addition to the duct tape on his mouth.Bosch is beaten by the other guards for his "betrayal" and is put into prison. A scientist called Lars notices this and attempts to contact Professor Thon. The guards, who are aware that the professor cannot be reached, are convinced by Berus that the entire situation is a test, in which they want to see their reaction. They take over control of the facility and capture Lars, Dr. Grimm and the other scientists after which they are put into prison as well.Dora comes to the facility a second time to speak to Tarek and is lured into a room by Berus, where he locks her up. The guard Eckert attempts to rape Dr. Grimm until Tarek stops him after he escapes out of the solitary confinement through the use of a screwdriver he found inside the "black box." Tarek knocks down Eckert and frees Lars, Bosch, Steinhoff, Dr. Grimm and the other prisoners. They manage to escape by removing one of the plates at the wall of one of the cells with the screwdrivers and flee. Meanwhile, Professor Thon hears Lars' desperate message in his voice mail and leaves for the facility. Steinhoff and Tarek stay behind to lock the way back in order to prevent Berus from coming after them. Professor Thon reaches the facility and demands an explanation from Eckert, after which the latter injures him by accident with a pistol. The fleeing prisoners are ambushed by the guards and trapped. Bosch, who could not keep up with the others, loses his sanity and kills Eckert with a fire extinguisher. Dora then escapes out of the room she was locked up in, and steals Eckert's gun. She injures one of the guards fighting Tarek and Steinhoff, leaving only Berus to fight them. Tarek incapacitates Berus, who is nearly choked to death by Steinhoff until Tarek convinces him not to kill Berus.The movie ends with a news break, confirming two deaths (Schütte and Eckert) and three injured (Thon and two others). Both Berus and Thon will be put on trial.In this movie appears Christian Berkel who will later play the role of Fritz Shimon Haber in Haber.
Output: Question: What does the taxi driver discover?, Answer: an invitation to participate in an experiment.