An enormous alien vessel threatens destruction, and Kirk answers with the best bluff in Starfleet history.
The Enterprise encounters a navigation buoy and then a vast alien ship that demands the crew be destroyed. Kirk invents a weapon called corbomite that would destroy any attacker, and gambles that the aliens will believe it while the crew waits in fear.
The story is a landmark for establishing Kirk as a captain who solves problems with wit rather than fire. Lieutenant Dave Bailey is a young helmsman whose nerves are tested. The twist reveals a very different intelligence behind the threat, and it ranks among the most admired early episodes.