During the entire run of the original Star Trek, Bill Shatner stuck to a long-established toupee style, which we call "The Jim Kirk lace". But there are a few examples within Star Trek's run in which Kirk's hair was altered. One such example, perhaps the most subtle, is the third season episode "The Paradise Syndrome".
In the episode, Captain Kirk loses his memory and is left stranded on a planet of primitive yet contended Native American Indian-like people. Kirk inadvertently becomes their god, Kirok, and also falls in love with Miramanee (pictured below).
Crucially, Bill Shatner's transformation into a softer, gentler character in the episode is also reflected by the subtle transformation of the toupee. It becomes less greasy and just a little bit more fluffy. The sideburns - are they real or not (probably not)? Does the hair look more or less realistic overall? Conversely, these kinds of dilemmas cleverly direct the viewer to never become too contented with Kirk's new reality - after all, fluffy toupee or not, the entire planet he is on faces destruction from an asteroid if Spock, up on the Enterprise, fails to halt it.
And on a side note - an interesting bit of trivia: Bill Shatner's moccasins are without the "lifts" that his regular boots contained to make the actor appear taller - thus, Bill Shatner appears noticeably shorter in parts of this episode:
Image sourced from Trekcore.com
Anyway, we think that "The Paradise Syndrome," written by Margaret Armen, is one of the highlights of the often dreadful third season of Star Trek and possibly of the entire series. Most notable is its portrayal of a surprisingly multi-dimensional and emotionally mature Captain Kirk (the episode ends not with the usual humorous banter, but with Kirk facing the death of his wife and future child), underscored by a uniquely soft and sensitive toupee style.