The Enterprise comes upon two adjacent inhabited solar systems that have had their planets reduced to rubble. Heading into the next solar system they receive a garbled distress signal from the Federation Star Ship Constellation. When they reach the solar system, they find that all the planets except for the inner two have been destroyed.
As they navigate through the debris field, they discover the badly damaged Constellation drifting in space. Sensors determine that parts of the ship are still habitable but the warp drive and transporters are destroyed and the bridge has been depressurized. Kirk, McCoy, Scotty and some red shirts beam aboard and discover that the crew is missing. While investigating the auxiliary control room they discover the ship’s commander Commodore Decker (played by well known character actor William Windom) slumped over the control console in heavy shock. McCoy revives him with a medication and Decker relates to them that some device of mammoth proportions, “miles long,” was destroying the fourth planet of the system when they arrived and so the Constellation attacked it with all it’s phaser weaponry but with the machine’s hull made of “pure neutronium” it had not effect. The Planet Killer counterattacked with a beam of “pure antiprotons” and disabled the Constellation. To save his crew Decker beamed them down to the third planet and stayed with the ship. After the Constellation could no longer move the device ignored it and went back to destroying the planets. Decker’s crew called him and begged him to help them as the machine destroyed the planet, they were on but he had no way to save them and this is what led to his breakdown. Kirk speculates that the device is a Doomsday Machine unleashed in some long-forgotten war that destroyed both sides, leaving the machine to travel on indefinitely destroying everything in its path and using the debris from the planets it destroys as fuel.
Kirk sends McCoy and Decker back to the Enterprise and stays along with Scotty and the engineering team to reactivate the Constellation. Scotty is tasked with getting the impulse engines working and the rest of the team attempts to get the main view screen of the auxiliary control room functional.
Meanwhile back at the Enterprise Spock is towing the Constellation along and intends to head away from the subspace interference associated with the Planet Killer and warn Starfleet that the device is headed for the most populous area of the galaxy. Communication with the Constellation is cut off by interference and when Commodore Decker reaches the bridge, he relieves Spock of command and orders the Enterprise to attack the Planet Killer. And of course, this goes very badly. In the course of delivering a series of totally ineffective phaser blasts to the hull of the device the Enterprise is caught by a tractor beam and is slowly pulled toward the maw of the Planet Killer.
At this point Kirk gets visual sensors back on line in time to see the Enterprise heading for annihilation. Scotty provides Kirk with impulse power and some phaser capability. Kirk attacks the Planet Killer and this gives the Enterprise the chance to escape. Kirk contacts Spock and orders him to relieve Decker. Decker escapes from an escort and steals a shuttle craft and despite pleading by Kirk flies it directly into the maw of the device. The explosion of the shuttle craft’s small engine damages the Planet Killer by a small but definite amount. Kirk theorizes that exploding the impulse engines of the Constellation inside the device might destroy the Doomsday Machine.
Scotty rigs a 30 second delay to provide Kirk with time to escape the Constellation before detonation. As the Constellation comes within a few hundred miles of the device Kirk pushes the timer and calls to be beamed out. But the transporter was damaged during the battle with the Planet Killer and we get the comical scene of Kirk getting closer and closer to destruction and anxiously reminding Spock he needs to be saved. Spock provides monotonous reminders to Scotty of the imminent demise of Kirk while the engineer works feverishly to repair the transporter’s something or other. And of course, Kirk makes it out with nothing to spare and his atoms scrambling in the air as the transporter manages to collect him together out of the hellish nuclear inferno set off inside the Doomsday Machine by the Constellation’s self destruction. We get some prattle between Kirk and Spock about the Constellation’s detonation which is like a hydrogen bomb, the 20th century’s doomsday device, being used to destroy a different doomsday device.
This is a great episode. The writer, Norman Spinrad, although not an author I preferred was a competent science fiction writer so he has crafted an interesting science fiction story. The Decker character is given a good part as the Captain Ahab trying to get his White Whale. Kirk gets to add a little humor to the situation of his transporter malfunction problem and he actually does this admirably. He even gets to tell Scotty he earned his pay. There really isn’t too much Shatner acting to mock but this episode doesn’t need it. I’ll call it a 10 // 0. This is as good as it gets for Star Trek.