This episode starts strong, ends strong, and the title alone makes it an instant classic. (-Spoilers follow-)
When the U.S.S. Enterprise encounters a space probe left behind from a federation ship, without hesitation they beam it aboard within feet of the Captain and First Officer. -You rarely see this kind of brazen arrogance as the series progresses, but when the wrecked probe flashes a green light, that really spells trouble. Very much like the first episodes of Star Trek the Next Generation, the the crew is overly dramatic and even Spock gets emotional. We learn some unique facts about the crew, including that Kirk "was a stack of book with legs" and was at one time a teacher! This episode features a ton of locations and more special effects by far than almost every other original episode to follow. You get the impression this is one of the productions that later inspired "budget cuts" and "fewer locations" because they really go off on sets in this one. The planet is possibly the best in series. The space view of the world is hot: a gaseous layered atmosphere that rivals some of the plain CGI planets of STNG.
I wont give away too much of the story, but it's a classic. Space phenomena mutates human into a veritable E.S.P. monster with awesome god-like abilities. Its amusing to watch Spock suggest that "killing him" is the only way -and he even requests a "phaser rifle" to help complete the task! Bottom line: A rich blend of Star Trek devices, plot twists, phenomena, and history. One of the best ever.