context: While reading a newspaper advertisement, taxi driver Tarek Fahd discovers an invitation to participate in an experiment, in which 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. He decides to join in.
  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, 12 prisoners and 8 guards, and are being observed by a team of scientists.
  In the experiment, 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 prisonerâs 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 SchÃ¼tte. 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 when the guards kidnap Tarek from his cell late in the night, order him to strip fully naked, shave his head bald and urinate on him.
  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 on the guardsâ 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 on, 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, who was forced to clean the toilet with his own clothes, pleads to the friendly guard Walther Bosch to secretly bring her a message. Berus intercepts Bosch, however, and tells Dora that everything is all right and refuses to let Tarek to see Dora.
  The situation becomes critical and more violence is involved. The prisoners are being abused and their self-esteem is drastically decreased by the guard's power. Most of the violence is directed against Tarek and they lock him up in solitary confinement inside a "black box" resembling a safe. SchÃ¼tte, protesting this, is beaten severely, bound and gagged with duct tape and forced to sit 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 confinement. Lars, a member of Thon's team, notices this and attempts to contact Professor Thon, who is attending a conference. The guards, who are aware that the professor cannot be reached by phone, are convinced by Berus that the entire situation is a test put up by Thon's team, in order to make the guards handle an exceptional situation. They take over control of the facility and capture Lars, Dr. Grimm and the other scientists, who are put into confinement as well, and gradually start their own prison in which they engage in brutally sadistic games with the total control.
  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. He is stopped by Tarek, who escaped from 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 the wall paneling of one of the cells with the screwdriver. Meanwhile, Professor Thon hears Lars' desperate message in his voicemail and leaves for the facility.
  Meanwhile, Steinhoff and Tarek stay behind in order to prevent Berus from coming after them. Professor Thon reaches the facility and demands an explanation from Eckert, who accidentally injures him 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 from the room she was locked up in, and steals Eckert's gun. She injures one of the guards while he is engaged in 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 film ends with a news break, confirming two deaths (SchÃ¼tte and Eckert) and three severely injured (Thon, Berus, and Bosch). Both Berus and Thon will be put on trial, Berus for multiple homicides and torture with rape, and Thon for allowing an illegal and unethical experiment.
question: What does the taxi driver discover?
answer: Invitation to participate in a prison experiment