I have a serious question: is Don Knotts supposed to be Don Knotts, or is he supposed to be the detective? The episode seems Knott to know!
There's a lot to like here - Don Knotts' series of disguises is great, his pep talks he gives himself in the mirror are hilarious, and the gang as always is in fine form. Great moments: Scooby trying to tell everyone he saw Don Knotts on the road behind them; Fred casually reading a book of ghost stories in bed in a (maybe) haunted mansion; Shaggy's ever more elaborate sandwich constructions.
The mystery itself sort of works too, but there are a few pieces at the end that really didn't come together for me, which is why I dinged this from a 7/10 to a 6/10. The big one is just the nature of Don Knotts' character. If he'd been the detective from the start, that would have checked out, but he's clearly "Don Knotts," and why would Don Knotts the actor be investigating the disappearance of the Captain? Another is, how is Don Knotts in the suit of armor? Seriously! I'm pretty sure he goes from in costume to in the suit of armor to back in costume over a series of quick cuts, and it doesn't make sense. Are there *two* Don Knotts in this universe? It's also confusing how Don Knotts was able to put together such a sweet dressing room inside the walls of someone else's house, but that's at least excusable because those parts are really funny.
A more minor complaint is that they've sort of given up explaining how the bad guys pull of their supernatural effects - I'd have loved to know how they did the ghost effect, since Don Knotts did it early on and then the two nephews did it at the end. Some crazy explanation about fishing line and rear projection - that was always a highlight of the classic episodes.