FRYEAN MYTHOS: *drama*
CAMPBELLIAN FUNCTION: *cosmological, psychological*
THE DOOMSDAY MACHINE is another "terror-of-technology" episode, though there's no attempt to imagine the nature of artificial intelligence, as in THE CHANGELING. The titular machine, sometimes called "the Planet Killer," is a gargantuan, nearly invulnerable cylinder that travels through space, breaking up planets and consuming their remains, regardless of whether or not the planets were inhabited. Kirk and company theorize that it was some alien race's Ultimate Weapon that went berserk, perhaps killing its makers and then continuing to follow its pre-programmed course of destruction.
The psychological interest here lies not in the machine, but in one of its victims. As the Enterprise ventures into a solar system devastated by an unknown force, they find one survivor aboard a shattered Federation ship: Commodore Matt Decker, a friend of Kirk's. Decker has almost been unhinged by watching his ship crippled and his crew destroyed by the Planet Killer, so while Kirk and Scott attempt to salvage the wrecked ship, Decker is sent back to the Enterprise. This proves to be an error on Kirk's part, for once Decker's on a functioning ship, he displays an Ahab-like determination to destroy the Planet Killer. He assumes command and tries to pit the Enterprise's inadequate resources against the space-behemoth.
DOOMSDAY is another of the series' "white-knuckle" episodes, concerned with the imminent peril of the mammoth menace and the possibility that an obsessed Starfleet officer may get everyone killed as a result of his own desire for a "do-over," or even a displaced desire for suicide, to join the dead crew he feels that he failed. William Windom gives a stellar performance as the obsessed Decker, though both Shatner and Nimoy play off him well. Decker seems to be the first indication that even a man who rises to the lofty position of starship-commander may prove unequal to the task; prior to this episode, the only psychologically flawed Starfleet officer was Ben Finney of COURT MARTIAL, and he was explicitly sent around the bend because he didn't realize his dream of becoming a ship-captain. There won't be too many Decker-like officers in future episodes, but the Roddenberry-verse always leaves open the possibility that power-- or even a lack of power-- can corrupt.