This ep was nominated for a Hugo Award for Best Dramatic Presentation at the 1968 World Science Fiction Convention. Norman Spinrad recycled a short story of his called “The Planet Eater” which was heavily influenced by Herman Melville’s Moby Dick. He convinced Gene Roddenberry that the material would be suitable for his TV show. The U.S.S. Constellation and its crew were destroyed by a “miles long” robot which consumes planets for fuel, leaving only a guilt-ridden/nearly hysterical Commodore Matt Decker (William Windom) aboard the “wasted hulk.” Capt. Kirk, Scotty, and a few crewmen beam over to begin repairs while Decker beams aboard the Enterprise. Kirk loses radio contact w/ his ship. The Planet Killer suddenly attacks the Enterprise, so ship is thrown off-course from the Constellation‘s position. Next, Decker seizes command from Spock- the story takes off! Decker recklessly takes the Enterprise into battle against the seemingly indestructible weapon. Kirk attempts to get the Constellation moving again to come to the aid of the Enterprise.
All our main heroes get something to do here! We see the new Engineering set created for Scotty; he gets a prominent role. Kirk works to repair the other ship, even getting his elbows dirty. McCoy (concerned/frustrated) tries to stop Spock from handing control over to Decker. Spock explains that Starfleet regulations allow it, but notice his side-eye and micro-expressions. Also, McCoy didn’t give Decker a medical exam yet (so can’t qualify him as “unfit for duty”). Sulu is sweating up a storm while trying to figure out whose orders to follow.
According to Windom, he had Decker compulsively twiddle w/ data tapes in his hand as an homage to Humphrey Bogart, who did the same thing w/ ball-bearings as Captain Queeg in The Caine Mutiny (1954). Those data tapes are gold and green, reminiscent of the colors of his and Kirk’s uniforms and hint at the tension between the two starship captains. Windom did not enjoy working on the ep; Shatner and Nimoy weren’t getting along at the time (giving the set a tense atmosphere). He purposely overacted; many years later, Windom realized that his character was a reference to Capt. Ahab from Moby Dick.
This is the most effects-heavy episode of S2; if you’re watching on Netflix (like me), you’re seeing the remastered version. When the series was digitally remastered (for 2007 DVD release), the upgrade required nearly 200 new effects shots. This was one of very few episodes to have its own score composed specifically for it; Sol Kaplan’s music was later used in The Immunity Syndrome, Obsession, and The Ultimate Computer. Many fans have noted similarities between the “planet killer” theme and the “shark” theme in John Williams’ score for Jaws (1975).
 The Doomsday Machine is a war story at many levels. It is the story of war spun out of control exemplified by a self-sustaining ancient device which seeks, destroys and digests whole solar systems. It is a story about military conduct aboard Federation ships. And it is, of course, a parable connected to the development and proliferation of nuclear arms.
 We’re back to some overwhelmingly grim tidings with this episode, a jarring reminder that there are some very dangerous threats out there in space. In a way, this can be looked upon as a precursor to the entire Borg threat introduced on the TNG series; on that show, something like the Borg was needed to shake the Enterprise and the Federation out of their complacency – or, shake them to the core, as is done to Commodore Decker here.
 The cosmic threat of this huge alien weapon, while exciting in itself, takes on a much more darker tone thanks to the presence of Decker on the bridge of the Enterprise. The whole plot seems to take a back seat, for awhile at least, to the strange, awful relationship between our psycho-damaged commodore and this unfeeling machine. Everyone else becomes an incidental side player to the conflict between these two, but, of course, it’s Decker, in his insanity, who creates a relationship; he no longer sees it as just a machine, a programmed robot, but as his personal devil.
-Excerpts from IMDB reviews