Kevin Murphy and Bill Corbett of "Mystery Science Theater 3000" and Rifftrax.com return to SF Sketchfest for a seventh edition of RiffTrax Shorts -- taking on some of the worst PSA, educational, scientific and promotional shorts of yesteryear, and riff them into submission.Joining the duo are guest riffers and original MST3K writers and performers Trace Beaulieu, Frank Conniff, Mary Jo Pehl and Bridget Nelson, frequent RiffTrax Presents team Cole Stratton and Janet Varney, and special guest John Hodgman and Paul F. Tompkins.