Kirk, along with psychiatric expert Dr. Helen Noel (Marianna Hill), investigates the methods of Dr. Tristan Adams (James Gregory), a penal colony administrator who is using unique technology to control and terrorize his patients. The story's tech device is a "neural neutralizer," which allows Adams to cause great pain in patients who disobey him. No points for guessing that at one point in the story Kirk ends up as one of the "patients."
Acting gets the job done in an episode like this. When your neural neutralizer is no more than a little plastic dome on the ceiling with a rotating light in it, you need acting to sell the idea of it causing extreme pain, loss of will, and submission to an authority. Shatner is enjoyable to watch in such a situation, though the true praise deserves to go to Morgan Woodward as the tortured Dr. Van Gelder, who has been forced into deep madness by Adams' "therapy."
The Trek franchise's first mind meld confirms Spock's suspicions that Van Gelder is a victim rather than the problem. "Dagger" is one of those episodes that looks at the side of Trek that rarely manages to rear its head—that of analyzing violent tendencies and the potential madness and corruptibility of people. And by the end, the little plastic dome is actually a pretty terrifying object.