Warner Bros has released a plot synopsis for the movie which, while not exactly detailed, has set Batman fans’ minds a-whirling. The official Warner Bros. 2019 editorial guide for the film reads:

“Joker centers around the iconic arch-nemesis and is an original, standalone story not seen before on the big screen. The exploration of Arthur Fleck (Joaquin Phoenix), a man disregarded by society, is not only a gritty character study, but also a broader cautionary tale.”

Joker will reach cinemas on 4 October, 2019.
Source.