In London, the friendly Professor Isaac Belfour sneaks onboard the Pussy Cats' bus and tells them of his rogue assistant, Mr. X, who used the professor's new formula to make himself invisible and seeks the paper that has the exact details of the antidote he's created. They agree to help the professor hide it, taking him along for their gig in Puddletown, England, to protect him from Mr. X, but not before he uses his newfound ability to capture three of the gang, and the others have to rescue them.
- This is the second major appearance of the Pussy Cats' tour bus.
- Sherry Alberoni as Alexandra Cabot
- Barbara Pariot as Valerie Brown
- Jerry Dexter as Alan Mayberry
- Jackie Joseph as Melody Valentine
- Casey Kasem as Alexander Cabot III, Constable
- Don Messick as Sebastian
- Janet Waldo as Josie McCoy
- John Stephenson as Professor Isaac Belfour, Mr. X, Mr. X Henchman