It's a great episode overall, but even as a kid (1970s, just when "nanu-nanu" was becoming the latest fad), I knew that vents going to outer space and having a path to the interior was begging for trouble.
Of course, the shuttle bay was shown to be pressurized and depressurized (imagine if those systems ever failed and there's a lot of oxygen in there) but the vent system is made to sound as if one could open it up without issue at any ol' time.
Garrivick was using his hands to cover the opened vent... how airtight are the vents and why can't the misty cloud just whiz right through? Once they knew the creature was in the vents, why couldn't they use the ship's computer to close all vents except for the ones that zigzag and go back leading toward outer space? How many vents interconnect with impulse engine disposal systems anyway, since they make it sound like everyone wants to get high by occasionally whiffing engine fumes because having separate, isolated vents exclusively for the engines was somehow a worse idea? (DS9 also had fun with vents in "One Tiny Ship" but by the 24tgh century they had force fields the way they didn't in the 23rd.)
Honestly, Scotty flushing the vents with radioactive materials would probably kill the entire crew in a rather gruesome manner, leaving the cloud on - forgive me - cloud nine... unless the vents were made with a metal (lead) that prevents radiation from going through the metal to poison everybody with? Of course, lead can lead to other unwanted problems...
"Obsession" actually is a great episode, it brings up Kirk's past, has a new kid learning the ropes and doing the same thing Kirk had, the use of instinct based on experience to out-think what is an adaptable but ultimately less intelligent life form (the cloud), how Kirk has to respond without ending up no different to the incorporeal corpuscle killer he's facing...
...but the vent problem always drove me as nuts.
Another fun note: I'm glad TOS had no continuity between episodes as such. As far back as "The Devil in the Dark", if not "The Man Trap", the "monsters" were given some depth, back story, instinct, reasoning, and intelligence. "Obsession", as with "The Immunity Syndrome" and others, doesn't "go there" with giving a perceived baddie some sympathy and remains a strong story no less. While leaving an opening for a prequel, if anybody wanted one. Maybe the cloud was a biogenic weapon devised to kill the borg - that explains everything right there and then and without diluting the "Obsession" story in the slightest.
But that's the other point about this show: TOS was not about strong inter-story continuity as such between episodes (VOY was no worse). It's about the philosophy, discussing human and societal nature. And that's where Man Trap, Obsession, Devil in the Dark, and numerous other episodes hold up equally well despite contradicting one another on the "baddies" being whatever they are or not. Because
the philosophy and human condition and telling them from multiple considerations were the point: Not learning something on
Mr Crater's Neighborhood or
Sesame Ship one day and applying it in all the subsequent episodes as a serial or arc or anything. Not when there are so many aspects of humanity to explore. As such there is no continuity, nor should there be. TOS was more metaphysical and exploratory. "Obsession" left plot points open and spent more time on Kirk's and Garrovick's baggage and rightly so. For "Obsession", it's not about how the cloud was created or why it exists. The cloud was there and killing people and the point wasn't to discuss what the cloud was about. (That and the idea some race of critters, maybe a species from a fluidic space dimension

created it and plopped it into our universe to go after anything organic, of which the borg and all the humanoid life in the Star Trek universe qualifies.)
That, and it takes more time to write a hundred episodes while remembering all the morals and continuity the prior ninety nine had introduced...
Still a solid 8/10
Side note: I just can't believe I just did a nitpicky in-universe stitching of the sort I was defending "Obsession" for not doing of such obsessive magnitude.
