Ok, there are a lot of Star Trek episodes that show alien civilizations with an unbelievably close likeness to Earth appearance and technology... even to the point of mimicking the Declaration of Independence. Those are big plausibility holes, but putting those kinds of things aside, episodes can stand up pretty well for themselves.
But there are some episodes where the basic premise is just so flawed, that it makes one put them at the bottom of the list for watching again.
OK, let's begin.
"The Mark of Gideon"
What? Population control is a big problem? I can understand that. So... you do your research and find that a certain infection is phenomenally good at killing off people. How to get it? Well, you find a carrier. Someone who survived the infection but still has the virus in their blood, which can infect someone else.
OK... So Kirk has something like HIV then. Nice. Yikes, stay away from THAT guy! Don't let him kiss any women!!
OK... here you go; you find that the captain of the Federation flagship Enterprise has had this amazingly rare virus. Invite the Federation for a visit and hope Captain Kirk coincidentally comes. Kidnap him through some clever ruse. Spend all this effort to duplicate the USS Enterprise decks just to confuse the heck out of Captain Kirk and hope he somehow falls in love with a woman who will steal his blood and infect herself.
Ummmm... couldn't you just appeal to the Federation and ask for some contaminated blood for research?
Stupid, stupid, stupid... one of the worst Star Trek episodes ever. If not for the unique vacant ship scenes spread throughout, I'd never watch it. Almost as bad an episode as "The Children Shall Lead."
But there are some episodes where the basic premise is just so flawed, that it makes one put them at the bottom of the list for watching again.
OK, let's begin.
"The Mark of Gideon"
What? Population control is a big problem? I can understand that. So... you do your research and find that a certain infection is phenomenally good at killing off people. How to get it? Well, you find a carrier. Someone who survived the infection but still has the virus in their blood, which can infect someone else.
OK... So Kirk has something like HIV then. Nice. Yikes, stay away from THAT guy! Don't let him kiss any women!!

OK... here you go; you find that the captain of the Federation flagship Enterprise has had this amazingly rare virus. Invite the Federation for a visit and hope Captain Kirk coincidentally comes. Kidnap him through some clever ruse. Spend all this effort to duplicate the USS Enterprise decks just to confuse the heck out of Captain Kirk and hope he somehow falls in love with a woman who will steal his blood and infect herself.
Ummmm... couldn't you just appeal to the Federation and ask for some contaminated blood for research?
Stupid, stupid, stupid... one of the worst Star Trek episodes ever. If not for the unique vacant ship scenes spread throughout, I'd never watch it. Almost as bad an episode as "The Children Shall Lead."