Rocky Horror Picture Show?

I just watched the movie... and the last half hour has got me totally confused. I know it's not really supposed to make sense, but I still have some questions. Is Riff-Raff, Magenta, and Dr. Frank-n-Furter aliens? And why did Riff-Raff and Magenta turn on Dr. Frank-n-Furter? What happened to Brad, Janet, and Dr. Scott?