This film, like the play that it comes from asked the audience to keep the secret of the Bat secret so I will do the same and so I'll only speak in generalities.
One of the grand old dark house films this is the story of the super criminal known as The Bat and his attempt to steal a great fortune.
This film isn't the best ever made. The passage of time has diminished its impact a bit. Part of the problem is that at times the film has to wrestle with its stage origins, things seem to stagnate and you become bored. However at other times this film soars with a visual style that has rarely been matched. The mask of the villain for example hasn't been equaled. Thankfully its always watchable.
Certainly worth a look.