They will discover what really happened back then with their favorite movie

It's not really clear what you mean. They will discover... by watching the movie? Or they will discover what happened during the making of the film? Or what happened to a video cassette/DVD?

Dear Elanguest: First of all, thanks for your answer.

They will discover by searching in the city and talking to persons what happened with the movie, because it never was released. It was shut down by a mobster (the film was almost complete and ready for the exhibition to the public but the mobster stopped the post-production)

Thanks!

"They will discover what really happened back then with their favorite movie" is probably the best way to arrange that sentence, but it's a difficult one, because there are so many variables that aren't clear to the uninformed reader. Without context, it's not at all clear what refers to what - it relies on the reader knowing who "they" are, whether they're the same "they" as the "their" at the e

True. The sentence might work if there is sufficient context around it.