One of the Star Trek's better "Monsters Attack" episodes is Obsession. Lays some pipe for Capt. Kirk and explains the origin of his nervous disorders. Introduces us to some new elements on the periodic table, including a substance even harder than the armor used for Outpost IV (Balance of Terror).
Plotline: Capt. Kirk's latent PTSD is awakened when one is killed, another critically injured by a fantastic hemoglobin-absorbing gaseous creature. We then get our backstory, chase the cloud across deep space, Enterprise attacks without result then fends off the cloud creature's counterattack. Cloud is repulsed with nuclear waste and flees. By intuition, Kirk sets course for the creature's nest. Then we resolve the conflict by destroying the cloud with the universal solution, an anti-matter bomb. Tension enhanced in the final scene with insubordination and battery on a superior officer by Ens. Garrovick.
It is apparent to all who have studied the rise of motor vehicles in the 20th century that any killer cloud story must have had its origin in Southern California of the late 1960s, in the days before the Clean Air Act and California Air Resources Board. Technology, such as the catalytic converter, would eventually dispel the real-life lethal smogs of nitrogen oxide and partially-combusted hydrocarbons that writer Art Wallace breathed on a regular basis.
Overall, a fairly good story, slightly above-average performance by the late Stephen Brooks as Ens. Garrovick. Incidental music recycled from previous Season 2 episodes. Original opticals only fair, almost cartoon-like, CGI a big improvement. Worth a look, but don't pay to see it. I rank it around number 32 of the 79 original Star Trek episodes.