The story of Investigation is about the brief and deep involvement between a man and a woman who are at war with each other. 
The man, Plamen, is detained as the only suspect in a fratricide case. But there is no direct evidence that he has killed his brother, no witnesses, no clues - the only chance the investigator has is to make the man confess. But the man is very strong and willful. He has been detained and interrogated daily for three weeks already, but he has said nothing. And he can be detained without conviction just for three weeks more - three more weeks of silence and he will be free. He can surely do that for his life. And then a new investigator is assigned. Plamen is almost humiliated by the fact that she's a woman. The woman, Alexandra, is the only female investigator in the department.