My version... although I have some serious doubts (SPOILERS)
What I understand after watching the movie is the following. I may be wrong of course, so any comment will be appreciated.
A long time ago, Daniel and Paul were close friends. Before commiting suicide, Paul sent a manuscript to Daniel of a novel he wrote based on Daniel's life. In a letter he said to Daniel he said he was allowed to do whatever he thought it was best with it. So, Daniel published the novel under a pseudonym.
A few years later, Paul's daughter (Anna Mouglalis) marries Daniel's son. Thinking he (Daniel) stole the novel from his father, and with the help of her friend Milla (Magdalena Mielcarz), she has an affair with Daniel only to blackmail him in the end.
I think she really got in love with Daniel, as after getting the money and splitting from him, she could just pretend to go on with her husband and life. After all, the blackmailer from Daniel's POV was Mila, not her.
However, she comes to Daniel once more, only to discover the truth about the book and realizing that Daniel knows that she is not Mila but Ewa. So, she realizes Daniel knows she is involved in the blackmail plan. And... she kills him and pretends he had commited suicide.
There are some questions though that I don't get.
1) Is Ewa the real daughter of Paul, or are her and Mila some troubled girls that use the rumor about the stolen book to blackmail Daniel?
2) What's the point of one of the last scenes when Ewa sees the picture of Paul and Daniel? Did they actually exchange names too? That would make no sense at all. Daniel's father would know his real name, there would be no need to go to Poland to investigate,...
3) I've read some comments that say that Daniel really commited suicide, but I think it is quite clear in the movie that Ewa is wearing a gun just before we hear a shot. Another question arises at this point, why did she had a gun in the first place? If Daniel haven't had discovered that she was Paul's daughter, she would stayed with him as she was in love...