The Enterprise's sister ship, the USS Constellation, is found severely damaged and adrift in space. The ship's captain, Commodore Matthew Decker (William Windom) is the only person left of his crew, the remainder of which had been beamed down to a nearby planet for safety. Unfortunately, nothing is safe from the unstoppable device roaming the region, which destroys entire planets, consumes the debris, and then moves on to the next planet. The death of Decker's crew has sent him into an insane obsession to destroy the device, even if it means suicide.
"The Doomsday Machine" sets the standard for all-out TOS tactical space action, with flawless pacing, a terrific score by Sol Kaplan, and lots of boldness. With Kirk and Scotty trying to make repairs to the Constellation, we have a great conflict aboard the Enterprise, where Decker takes command of the ship while Spock and McCoy are initially powerless to stop him from launching a suicide mission. Spock's calm but firm action in the face of this adversity is an absolute joy to watch unfold.
Keeping in tune with the series' social commentary is the low-key allegory on nuclear weapons—which is implicitly present, but never threatens to eclipse the story.