In "Der blonde Affe", Jürgen is the art-dealer Jan Fleuten. The film starts with a scene in his studio in an old warehouse at the harbour of Hamburg, where he is doing something suspicious to a madonna statue. It is a wet and stormy autumn night. Fleuten goes out into the night to watch the goings on in a house. Inside a young man, Andi Prolsch, and an older woman, Elaine Troost, quarrel about money. Elaine is the owner of an art gallery but only on paper, the real owner is Andi's father. Andi, who works in the gallery, blackmails Elaine by threatening to tell his father about some shady business, that is going on in the gallery. Elaine refuses to pay anymore and in the following fight, he pushes her through a glass door and she falls into the garden.
In the next scene the police are all over the house and garden, because the dead body of Elaine has been discovered in the bath by her daughter Gabrielle, who lives on the upper floor of the house and also works at the gallery. Her theory is, that Elaine drowened in the bath while being drunk, which she was frequently. The two policeman, who interview Gabrielle are the older Bellmann and Graf, who is soon fascinated by Gabrielle. When they are gone, Gabrielle remembers the happenings of the night: she came downstairs, when she heard the glass of the door break and helped Andi, who seems to be her lover or at least a good friend, carry her mother back indoors. At this time, her mother is alive, though quite drunk. Andi leaves the house, shouting, that Elaine still owes him money.
The police decide to do a post-mortem, because there are wounds on the body not consistent with simply drowning. A neighbour has seen a visitor with Elaine on the night of her death, a visitor wearing long hair in a pony-tail, who could have been a man or a woman. Another neighbour, who was some time ago accused of poisoning Elaine's dog, will have to be interviewed.
Meanwhile Gabrielle and Andi meet and decide, not to talk about the quarrel between Andi and Elaine in order not to to draw attention to the shady business going on at the gallery.
Bellmann and his boss go to have a look at the gallery and meet the manager. They tell him the results of the post mortem: Elaine was drowned by somebody, so it is a murder case, not an accident. After they leave, Andi turns up and pretends to be surprised by Elaine's death and discusses with the manager the details of some illegal business, that is soon going to take place,
Graf is sent to talk to the dog-poisoning neighbour but has no success. He goes on, to talk again to Gabrielle. Graf shows her a picture of Fleuten, which they have found in Elaine's flat. Gabrielle explains, that he was once Elaine's lover but ended the affair. Fleuten was once also attached to the gallery but quit after difficulties with the manager. Elaine was once a successful singer, Elaine Carnet, before she married. She got divorced and had been very lonely, especially after Jan Fleuten left her.
When Graf tells his colleague about the interview, he is accused of falling in love with Gabrielle and being unprofessional since she is a suspect in this case.
The police find evidence of the quarrel on the glass door and in the garden and another neighbour reports, that she had seen Jan Fleuten on the night of the murder. Enquiries by the police later show, that Fleuten has a history of smuggling and forging art.
The older Prosch, Andi's father and the real owner of the gallery arrives, but not so much because of Elaine's death but to finalize arrangements with the manager for an illegal deal with the Russian mafia. Elaine may have done some business on the side, but Prolsch Sr. does it on a bigger scale: the value of the statue, that the Russians are going to sell is 2 million marks. It becomes clear, that Prolsch does not think much of the abilities of his son Andi.
Fleuten is up to something with two associates in his studio. When he sees the two policemen approach, they get into Fleuten's boat. The policemen take some boat nearby and there follows a wild chase through the harbour, during which Fleuten manages to put his two friends on land without the police noticing. Afterwards, he goes back to his warehouse and peacefully awaits the policemen.
Fleuten's studio is full of statues and pictures showing monkeys. He explains, that he had a childhood trauma about monkeys and that he decided to face his terror by surrounding himself with monkeys. That it is a good thing, to be afraid, because it keeps you on your toes. He is particularly fascinated by Hanuman, the Indian monkey god. One of the tales about Hanuman, explains, that all things have two sides: when Hanuman kills one of his mythological enemies, at the same time, he frees that enemy from a curse by killing him.
Andi and Gabrielle have another meeting to discuss, how to go on. Gabrielle convinces Andi to keep a low profile and stay away from the gallery, especially when the deal with the Russians takes place, as a person with a pony tail has been seen in Elaine's house.
Bellmann and Graf discuss various suspects over some beer and it turns out, that Bellmann doesn't want to go home because he can't stand his wife. Then we are shown another scene from Gabrielle's memory of the night of the murder: Elaine and Gabrielle talk, while the drunk Elaine is sitting in her bath. Elaine has read Gabrielle's letters and found out, that Fleuten and Gabrielle are lovers. Elaine is furious about this, she tells Gabrielle, that Fleuten will drop her just as he dropped Elaine and that she still wants him for herself. Elaine threatens to go to the police and report all the illegal business going on at the gallery to implicate Fleuten, even though that would mean, that she would accuse herself too. Gabrielle is besides herself and attacks her mother and eventually drowns her.
When Gabrielle wakes up from this nightmare, we see, that she has actually been sleeping at Fleuten's place. Fleuten comforts her and assures her, that he loves her. He knows, what Elaine could be like, when drunk. During the conversation, there are hints, that the two have a plan.
Next morning, Graf visits Gabrielle again. Gabrielle tells him, that the mysterious pony-tail visitor might have been Andi. Gabrielle tries to seduce Graf to hear more about the plans of the police and distract from her packed suitcase. She almost succeeds but Graf notices a tatoo of Hanuman on her shoulder.
When Graf returns to the office, Bellmann knows at once, what he has been up to and gets furious about this unprofessional behaviour. At that stage, the dog-poisoning neighbour arrives, to make his statement. He has a habit of watching Gabrielle undress and has seen, that a man with pony-tail and Gabrielle carried the mother indoors after the accident with the glass-door. Meanwhile Andi by chance sees Fleuten and Gabrielle visit a travel agent, he gets suspicious and tries to follow them but is involved in a traffic accident, which also leads to his identification as the pony-tail visitor. He tells the police, what he knows about the night, when Elaine died, because he is realizing, that Gabrielle has not been honest with him. Gabrielle and Fleuten go ahaead with their plan: Fleuten has made a copy of the statue, that the Russians are going to sell. His two friends pretend to be the Russian dealers, get the money and flee.
The old Prolsch is furious that Andi has told Gabrielle about the deal in the first place. They follow Fleuten and Gabrielle to the airport, all of them followed by the police.
Just before the flight is called, Fleuten is shot by Andi. Gabrielle , who has been waiting in the lounge, slowly goes on on to the gate, hoping that Fleuten will catch up with her. The police arrest Andi and his father and Graf is sent after gabrielle. He finds her at the gate, the plane has left. She asks Graf "Why didn't he come?" and Graf answers "He tried to come". Gabrielle is also arrested.