Kirk is obsessed with trying to kill a cloud-like vampire creature similar to one that killed Kirk's commanding officer, Captain Garrovick, 11 years earlier.
Air Date: 12/15/67
Director: Ralph Senensky
Around 2009, I finally grabbed all three seasons of TOS on DVD which enabled me to properly binge the series. This is one of the few that I had never seen in syndication. It's a great episode that adds some back-story to Kirk.