This is a kind of interesting episode - in this we discover that Kirk is haunted by an incident from twelve years earlier, when a strange gaseous creature, that was causing strange vampire-style deaths aboard the U.S.S. Farragut. Kirk, serving as a junior officer on the ship, had the creature in his phaser sights but hesitated to fire and has blamed himself for the deaths ever since.
When a junior crewman from the Enterprise similarly hesitates to fire upon the creature when the Enterprise comes across it again, Kirk is unnecessarily harsh. But as the Enterprise crew battle the creature, it becomes apparent that Kirk's hesitation years earlier would have made no difference to the outcome at all.
Interestingly, Spock's blood is revealed to be based on copper rather than human iron, which gives the gaseous creature bad indigestion and allows Spock to come into contact with the creature without danger. Though one minor niggle is that the creature is able to gain access to a presumably airtight starship from outside in space.
Finally, Kirk and the junior crewman face the creature alone and destroy it with a dose of ant-matter. And they all lived happily ever after ...
Overall, quite a tense episode that may not mine new dramatic material, but is nonetheless effective as a thriller.