Am not going to be as negative as the previous reviewer (while agreeing with all their issues with it) or go as far as to cite "The Diabolical Disc Demon" the worst Scooby Doo episode ever. If the question "what is the worst 'The Scooby Doo Show' episode", this would be a worthy contender in my view. This is not a new opinion by the way, this was an episode first seen before turning into a teenager and even then it did very little for me. Have seen it a few times over-time to be fair to it, but it just doesn't click with me.
"The Diabolical Disc Demon" to me is definitely one of my least favourite episodes of 'The Scooby Doo Show', and it was sad to me as music-themed episodes for any show always intrigue me and the Scooby Doo franchise has done some worthy music-themed episodes and films. "The Diabolical Disc Demon" has its moments but it is to me the first below average episode, "Jeepers It's the Jaguaro" was very disappointing too but at least it had interest points and had a memorable monster and at times tone.
Certainly there are good points, no episode of 'The Scooby Doo Show' is irredeemable. With one exception, the animation has vibrant colour, lots of atmosphere and great attention to detail. The music became more elaborate in Season 3 and had more of a groove, was more elaborate, showed that things were progressing and built upon and had more variety. Liked the grooviness of the 70s music and the theme song is a classic.
Some nice moments of humour with ever loveable Shaggy and Scooby and generally the voice acting is solid.
Like a number of other Scooby Doo fans, having seen quite a lot of dislike for the episode and the villain when the message boards here were still active, the villain is a real disappointment. A strong contender for the worst of the show, though the villains in "Don't Go Near the Fortress of Fear" and "The Warlock of Wimbledon" were pretty poor as well. The phantom is neither sinister or fun, actually found him on the wrong side of goofy and difficult to take seriously. Such a weird and derivative look and Michael Bell overdoes it, embarrassingly so at his worst, and doesn't have any menace or sound like he was enjoying himself. The motive is a pretty cliched one and nothing new is done with it and the Shaggy and Scooby encounters with the phantom are too heavy on coincidence and feel like repetitive padding.
Furthermore, the mystery is extremely weak, very thin on the ground and very bland. Not to mention ridiculous. Didn't detect much energy, very by the numbers instead, nothing comes over as creepy or surprising and there is no risk taking. The clues are far too few, barely any actually, and the incriminating one is so obvious (as is why the reason for the phantom being so desperate to get it back) that it defies belief that it took the gang so long to decipher it. As a result the episode is the biggest example of "it could only have been one person". Only Shaggy and Scooby are used well of the gang, the others are relatively underused. The writing has moments, but it is only with Shaggy and Scooby where it felt like the writers were having any kind of fun.
On the whole, below average and a real disappointment. 4/10.
