This is kind of a weird one. One of those mysteries where you read it and you're all "AH OK, good mystery, I get it." but then upon reflection, the details kind of fall apart. Sort of a reverse Keyser Soze.
A Rock 'n' Roll Zombie has stolen Steve Stringbean, singer for The Electric Pickle. Fred constructs a plan to capture the Rock 'n' Roll Zombie whereby Shaggy & Scooby perform as the singers of the band. Fred's feeling is that this will (for some reason) lure the Zombie out. And it does, they capture him, it was the security guards all along, yay. The end.
BUT if the security guards were holding the singer for ransom, why would they care if Shaggy & Scooby were singing with the band? There's absolutely no reason for one of them to put on the costume again and risk getting captured. AND NOT ONLY THAT, but why did they need to create the Rock 'n' Roll Zombie in the first place? Why not just capture the singer after the show, tie him up in the basement and sent out a ransom note, old-school style. I guess maybe it's fun to imagine the security guards inventing the Rock 'n' Roll Zombie out of a desire to be the center of attention for once.
ALSO WORTH NOTHING: there are 2 pictures in this book where it is very clear the artist was having a lot of fun lingering over Daphne's legs. Like they're all contorted in awkward model poses? I get it dude, Daphne's smoking hot, but get a room this is a kid's book.