The movie follows Rise of the Planet of the Apes, and Dawn of the Planet of the Apes in the ongoing Planet of the Apes saga. War for the Planet of the Apes is being directed by by Matt Reeves who directed 2014 Dawn.
The two movies have been hugely successful with the Apes pretty much stealing the show from their human counterparts. Harrelson is rumored to be playing a character called The Colonel. We do not know the details of the story but if you have seen the first two and the original 1968 version of Planet of the Apes then you have a pretty good idea where the story is headed. With War in the title we are expecting lots of action.