This was not only one of the scariest shows of the series, it was one of the scariest all around. The steady, implacable approach of the invisible whatever, the bizarre "negative effect" when the victims were consumed, and, most of all, that intense music, jacking the tension up to the screaming point. The scene where Hadji was left stranded, and snatched up just in the nick of time ("Once around the park, fellows!") was great, as was Jonny tearing out to rescue his panicked dog. And I thought Dr. Quest must be the bravest man in the world to face that thing alone.
I didn't mind it when the invisible became visible--after all, as they pointed out, in order to destroy it, they had to have a clear view of it, and if that wasn't the creepiest-looking thing I've ever seen....
Someone else obviously agrees with me. Years later, I was watching the epsisode "The Energy Eater" from "Kolchak: The Night Stalker". Also an invisible thing, they got an image of it by putting together dozens of X-rays that had been in a room the monster rampaged through. When they finally pieced them all together, I yelled out loud, "That's the monster from Jonny Quest!"
You can't keep a good monster down!