The main gist of the story is that it is a love triangle between three of the characters. The Phantom (whose name is Erik, as you will learn if you read the book by Gaston Leroux), Christine and Raoul. Erik loves Christine and teaches her how to sing, Raoul is a wealthy aristocrat who is also in love with Christine (as well as Christine's childhood friend). Christine is in love with both men and can't decide who she should be with.
The setting of the story takes place in the Paris Opera house, employees of the Opera house believe that there is a Phantom haunting the opera, in reality The Phantom is Erik, a genius disfigured at birth who lives in the lowest level of the Opera house, Erik uses trickery and illusions to seem like a ghost.
The time period of the story is in the late 1800s.