At the beginning, Sargon tells Kirk that if he leaves Sargon to perish, then all of mankind must perish as well. However, at the end, Sargon decides to go to oblivion (perish), without giving any kind of benefit or aid to Kirk and crew, so whatever would keep mankind from perishing has clearly not been averted, but nobody seems to be concerned that mankind is now doomed.
The aliens all speak with an echo, after inhabiting the bodies of the crew. There's no reason for this - they should have been speaking normally, as the crew would do.
When the two security guards fail to beam down with the rest of the team, Kirk asks Scotty if the security guard (singular) is still on the ship, just after Spock and just before Scotty each refer to them as security guards (plural). However, the singular noun 'guard' is also used a collective term for a guard team.
The usual rule is that when beaming down from the transporter room it is necessary to be standing directly on the plates in the floor. The two security guards are not standing fully on their plates and indeed neither of them beams down - although the reason given for this is that Sargon does not want them to do so. The other possibility is that the security guards stood where they did was to make it easier for the post-production staff to insert the transporter effects, without having to deal with the difficulties of fading the four principals out, while retaining the arms/bodies of the security guards behind the four that transported.
Thalassa tells Dr McCoy that only she and he would know that Anne has not returned to her body. However, the secret would have been given away the first time she spoke, because whenever Sargon or one of his people spoke, there was an echoing sound and amplification of their voice due to their being a highly-advanced and powerful species.
Thalassa tells Dr McCoy that only she and he would know that Anne has not returned to her body. However, whenever any of Sargon's people speak, their voices have a loud echoing quality, because they are only occupying the bodies of the Star Trek crew and their voices are coming from their powerful essences. Anyone within earshot would immediately realize that Thalassa and Hanoch were still occupying the crew's bodies.
Kirk and his crew do not believe that fully functioning android bodies can exist, apparently forgetting their encounters with such beings in What Are Little Girls Made Of? (1966) and I, Mudd (1967).
It is never made clear why the aliens depended on taking over living bodies in order to construct android replacements, rather than simply leave one or more androids in their shelter, ready to be occupied at the appropriate time.
Even more, the aliens could have been given simple robots to control from within their spheres, and used them to build more and more complicated androids until they were satisfied.
Even more, the aliens could have been given simple robots to control from within their spheres, and used them to build more and more complicated androids until they were satisfied.
Sargon has been telepathically communicating with the Enterprise crew since the ship came within range of his planet. But when Kirk at first orders Spock to stay behind, Sargon drains the ship's power to induce Kirk to change his mind. This would seem unnecessarily passive-aggressive; Sargon could simply instruct Kirk to bring Spock along.
Dr. Ann Mulhall wears a red tunic, but she is described as a astrobiologist, which should put her in the Sciences division and have her wearing a blue tunic.
Kirk at first orders Spock to remain behind on the Enterprise, claiming that the situation is "full of unknowns" and that he can't risk both of them. But the two of them have beamed down into situations full of unknowns dozens of times previously.
In the transporter room, Kirk doesn't know who Mulhall is. She has the grade of Lt Commander, so should be known to Kirk.
When Kirk is convincing the crew that the risk is worth the reward of hosting the alien minds, he asks McCoy if he wished the first Apollo mission hadn't reached the moon. In fact, Apollo 1 (also known as AS-204) suffered a catastrophic fire, killing all three crew members--Gus Grissom, Edward D. White, and Roger B. Chaffee. It was never intended to reach the moon, but only achieve Earth orbit.
Sargon says, "... the records of our travels were lost in the cataclysm that we loosened upon ourselves." The correct word is "loosed".