STORY LINE (in english):
Sina Wolf is a sixteen year old student from a rich family in Kiel.  Near the bicycle racks at their school, she is asked out for a date to go get ice cream by her classmate Michael Harms, who has a crush on her.  Although they have been friends for a long time, she' avoids an unwanted romance from him and she suddenly gives him the cold shoulder.  She says that she has too much work to do.  Sina has a secret.  She has had an intimate relationship with her teacher Mr. Fichte for some time.  

When Michael followed her unnoticed one day to a lake on his bike or did he stalk her?  Anyway, he sees her walk out onto a pier.  He moves closer through the woods.  A car then drives up and the teacher gets out and she runs up to hug him.  Michael appears surprised.  He almost even begins to vagely smile.  Then, as he continues to watch they row away in a boat to another part of the lake, undress and make out. happens to witness sexual acts between the two of them, which may have seriously injured him mentally.  He becomes jealous and very hateful.

He races away on his bike and ends up at Sina's house.  There, he mocks the idea that she had schoolwork to do, etc.. and he says he saw what she did.  Right then, her father drives up and goes into the house.  Then, Michael tries to blackmail Sina and demands sex from her under the threat that he will make this criminal relationship public.  In order to protect herself and her teacher, Sina sees no other way out than to comply with Michael's request.  

During class.  Michael can't concentrate.

Inge approaches Mr. Fichte at school.  She was at risk of being transferred.  So she says to him that she saw him and Sina at the lake and she knew about the relationship between the married teacher Mr. Fichte and their school friend Sina.  

Michael may have informed his classmate Inge  about the relationship between the married teacher Mr. Fichte and their school friend Sina.  Inge may even be telling his wife, a teacher also there at that school, Mrs. Dr. Gisela Fichte.

After school both drive together into a forest and she doesn't like his advances.  She then picks up a stone and kills him.  

Fearful, she adds more evidence' to try to make it look like rape against her.  She rubs her clothes on the ground and yells for help and she tells the police that Michael wanted to save her from being raped and that the perpetrator killed him, instead.  

Back at school, a kid gives Michael's notebooks to a teacher.

The description of the perpetrator fits a man who has been making headlines in the newspapers as a rapist for a long time.  

Inge is now blackmailing Mr. Fichte and thus gets the solution to the next math work that she writes and passes with that math teacher.  That math teacher actually is Mr. Fichte's wife, a teacher all at the same school.

Mr. Fichte and Sina meet at the lake again.  As they hug, he says to her: "We must now be cautious since all of this has happened ... a commissioner from Kiel is speaking to them about this.".  In return, she seems speechless.

During the investigation, Kiel police senior commissioner Finke learns of this teacher-student relationship and the boy at the school. 

The boy's father is dead and his mother is too much of an alcohlic to be concerned, so his uncle is mostly the boy's guardian.

At the boy Michael's funeral, his mother and uncle attend, as well as senior commissioner Finke, Mr.Fichte and his wife, Inge and Sina.

Sina is pulled out of class one day by Finke to go to the police station to identify a now dead man they think that could have attacked her, according to her original rape story.  She lies and says it was the man that attacked her.  When it was also determined that the alleged perpetrator was not possibly a suspect during the time in question, her teacher Mr. Fichte was then suspected. 

Mr. Fichte's wife mentions the writings in Michael's school book about him and Sina.  And how the police could get a hold of those writings.  Later, she mentions that they both could move to Lübbecke and work at an all boys school.

Confronted with accusations of taking advantage of a dependency relationship, he tries to end his relationship with Sina.'  But, she doesn't want to end it and she keeps trying to meet with Mr. Fichte.  He continues to call Sina. 

Finke and his police assistant Dressler are outside of Sina's house.  At times, she looks like she already knows.  They are possibly trying to catch Mr. Fichte going there or to see if she goes to meet with him.  Finally, while Sina's parents are away in places like Berlin, Mr. Fichte goes to her house to meet her.  She offers him something to drink. She says she loves him.  He says she loves her and they go to bed.  Mr. Ficht them leaves. Dressler then goes to her house and tells her that the man she claims was in the rape was not even in Deutschland.  Meanwhile,  Finke takes the car and follows Mr. FIchte away from her house to a field.  Finke then suspects Mr. Finke of killing Michael.

This girl Sina, also gets caught up in contradictions with the police. Under questioning by Finke, she says she loves Mr. Fichte but won't protect him..  

With tears in both ot their eyes, Mr. Fichte and Sina are pressured to stop seeing each other.  His wife and everyone else pressures him to stop visiting her.  Other girls at school begin prank calling her and joking about their relationship. 

Desperate, writes a note about dying and her parents find it and call the police. They try to find out where she is.  They go to the lake twice.  The second time with Mr. and Mrs. Fichte. 

She had fled 'to the lake, which was often the place of the secret meetings with her and Mr. Fichte and tries to shoot herself with her father's gun.  Since she couldn't manage to reload the weapon,  They find her still alive.

SOURCE: (updated)
Wikipedia Germany, Wikipedia the free encyclopedia
-- https://de.zxc.wiki/wiki/Tatort_(Fernsehreihe)#Liste_der_Tatort-Folgen
(see: 'Main article : List of crime scene episodes')
Book by Hrbert Lichtenfeld