The film suggests that Renfield was Jonathan Harker's predecessor as Count Dracula's agent in London; it is implied that this is the reason for his madness. He tries to persuade Mina to stay out of Dracula's grasp out of jealousy, angry that Dracula plans to give her immortality instead of him.
That version of Dracula saw Dwight Frye take on the role of R. M. Renfield, a servant to Dracula devoted to his master to the point of insanity. Now, nearly a century later, Renfield finally has his own movie.