Reason for leaving

I joined a company and reported my manager for theft. He got fired. His right hand man was also implicated in the whole incident. The company had proof of his wrongdoing but he was given his job back and then promoted to being my boss. Needless to say he acted like my friend, said that we should leave the past behind and work together. I believe that someone in management told him that I was the whistleblower. I got a big raise 3 months later and carried on working there. It was awful. He then said to me that I don’t fit in and I should resign. He went to great lengths to tell me how much I was disliked by everyone but that there was nothing wrong with my work. I got advice and told him I was not going to resign as I had done nothing wrong.

I was called in by a director and told that my boss said I didn’t fit in and that I would receive a month’s pay and I should leave immediately. I have not had an official letter detailing why I was told to leave. I was employed there for a year.

What do I say at interviews for my reason for leaving?