"Obsession" provides Kirk with his own great white whale episode, though in this case it's a great white Cloud. And that's a vampire Cloud no less, one that can travel through time and space, one that kills by sucking the red corpuscles out of its victim, unless you're Eddie Paskey in which case you come back later. The plot proceeds in a straightforward manner yet never uses any sort of flashback (though to be sure such are notoriously scarce in TOS). The material is far from original, containing recognizable if not recycled elements of "The Immunity Syndrome," "The Doomsday Machine," and "Wolf in the Fold" among others. Sadly the story starts with Kirk's titular (and mildly interesting) emotional block regarding his early career aboard the USS Farragut but dissolves into a space-hopping (and considerably dull) "bug hunt."

Somehow the main players summon enough enthusiasm to phone in their performance. Shatner has a head cold that effects his performance audibly and likely explains his listless tone. McCoy and Spock breathe a little life into it while considering the condition of their captain in rare cooperation. But mirroring the cast effort even the episode production seems unusually lackluster, with rough editing and weak attention to detail (except the cool antigrav!). The Cloud looks like well, a puff of smoke. Needless technobabble like "dikironium" only serves to confuse the matter. And why is McCoy wearing an engineering badge?

As "Assignment:Earth" favors Gary Seven, as "Lights of Zetar" dotes on Mira Romaine, so is this episode overly fond of Ensign Garrovick. (One might even say, "sickly sweet.") Why? No one knows. We never see or hear from him again. Frankly the character is only interesting compared to his dead father, Kirk's old captain, since throughout the episode he is petulant and disobedient. Striking Kirk risked the life of his captain, himself, and his crew, hardly behavior to be rewarded. (Maybe that's why we never see or hear from him again!)

The show does have a few good lines like "thank pitchforks and pointed ears" and "it's that green blood of his!" The title better applies to repeat viewers though, since this episode hasn't aged well. Why would the Cloud try to flee if attacking was a good idea? If attacking wasn't a good idea, why did it turn and attack after getting away (this at Warp 8!). And is it really acceptable for a starship commander to obliterate a planet to destroy a "monster?" All in all, this adventure is nothing to obsess about.