10 Most Important Episodes Of Star Trek: The Original Series
3. The Doomsday Machine (2.6)
While it may not carry quite the same amount of weight today as it did back in the 1960s, The Doomsday Machine was an episode that provided strong commentary on the concept of mutually assured destruction in a time period when the prospect of nuclear conflict was constantly looming during the Cold War. (Five years to the day before this episode aired in October of 1967, the Cuban Missile Crisis was an ongoing event.)
What's more is that it did it in a brilliantly subtle way, in addition to being a brilliantly tense episode that you could truly feel the scope of.
The episode follows the Enterprise's discover of a "planet killer", an automatic, self-propelled spacecraft with a path of unmitigated destruction in its wake. The ship goes into red alert as they scramble to figure out how to subdue the machine and, following the ultimate sacrifice made by a member of their sister ship and apparent survivor of the planet killer, Matt Decker, are able to disable it before it could destroy any more planets in our galaxy.
It can be inferred that the device is from an entirely different galaxy and, one could only assume, killed all or most of its creators and that galaxy's inhabitants before moving on to ours. Not only is this fitting symbolism for the aforementioned mutually assured destruction, but also serves as a cautionary tale of what can happen when man creates something devastating, only to have it escape their control (i.e. nuclear weapons.)