While on a planet for a survey mission, Kirk falls in a pit and is knocked out. He finds his way out again, and returns to the ship, but after they have left the planet, strange malfunctions start up all over the Enterprise--dangerous malfunctions. What is the cause? The malfunctions coincide with times Kirk has a terrible headache and leaves the bridge, so Spock suspects him. Spock turns out to be correct, and uses a mind meld to help Kirk fight off the aliens that have been compelling him. Once the aliens realize that the Federation means no harm, they are friendly and peaceful, and intend to set up diplomatic relations.
Sounds like an interesting enough plot, if too simply resolved, but there's one problem: the story reveals the trick to the reader on the first page, and we have to spend the rest of the story watching Kirk go off to cause mayhem while Spock stares at him and goes "Hmm...". There's no mystery at all for the reader, very little of one for Spock, and also very little tension about the ultimate resolution.
A minor note: this story was written after The Animated Series aired, so we have a reference to life support belts, a very useful innovation introduced in that series.