Kirk, Spock, and McCoy are brutally used by an alien species in order to help another alien learn a sense of self-sacrifice that can save her people.
The Empath tends to be one of those episodes fans love to hate, and I often wonder why. Its often called a "cheap" episode because the main set is a dark soundstage with black curtains and spot lighting. This makes no particular sense, because there is a lot of apparatus present and because a typical planet set for a Star Trek episode is merely a sound stage with a colored light backdrop representing a sky, some re-used foam hills and sand, and some dried out sea grass. This episode HAS one of those soundstage planets AND the main "dark" set. In addition, the make-up for the Vians is some of the most elaborate in the series. I bet "The Empath" cost as much or more to make than "Amok Time".
Other criticisms center on a pointless story, and I don't get this either. The main protagonist, Gem, is required to learn that sacrifice is a quality necessary for a species, and to prove it, she must alleviate the symptoms of Kirk and McCoy after they are brutally tortured by the Vians. The actions of Spock and McCoy show that altruistic behavior themselves, and Kirk's final over-acted speech even drives home the message that the Vians themselves need to learn the lesson that they are hoping to teach. I am not particularly a fan of mimes, but the fact that Gem is a really mute is necessary to the entire point here, she must see and feel the actions around her. This is a much bigger theme than many episodes where the allegory is more obvious, like "The Squire of Gothos", for example, where the take away message is "don't play with people, son". Some say the Vians could have found a better way to test Gem's people, but then again, the Kelvins could have thought of a simpler way to get back to Andromeda in "By Any Other Name", for example. The allusions are a little heavy in "The Empath", but they don't have to be as "religious" as some of the references in this episode make it out to be. Lots of recent work has shown that altruistic behavior is necessary and sometimes learned in many species of social animals on Earth.