Created by Gene Roddenberry, the science fiction television series Star Trek (which eventually acquired the retronym Star Trek: The Original Series) starred William Shatner as Captain Kirk, Leonard Nimoy as Mr. Spock, and DeForest Kelley as Dr. Leonard "Bones" McCoy aboard the fictional Federation starship USS Enterprise. The series originally aired from September 1966 through June 1969 on NBC.
This is the first television series in the Star Trek franchise, and comprises 79 regular episodes over the show's three seasons, along with the show's original pilot episode, "The Cage".
After the show's cancellation, Paramount Television released Star Trek to television stations as a syndication package, where the series' popularity grew to become a "major phenomenon within popular culture." This popularity would eventually lead the Star Trek franchise to expand its catalog to include five more television series and twelve Trek motion pictures.
On stardate 4202.9, following a trail of destroyed solar systems, the Federation starship USS Enterprise responds to a distress call, finding their sister ship, the USS Constellation, adrift and heavily damaged in a system whose two inner planets are still intact. Captain Kirk beams over to the Constellation with Chief Medical Officer Dr. McCoy, Chief Engineer Scott and a damage control team to investigate. Mr. Scott reports that the ship's warp engines are damaged beyond repair and the weapons exhausted. There they find the ship's commanding officer, Commodore Matt Decker – the sole survivor, half-conscious in the auxiliary control room. Meanwhile, an incoherent Decker can only mutter about something attacking his ship as McCoy treats him.
The logs reveal that the ship investigated the breakup of a planet and was soon attacked by an enormous machine with a conical shell miles in length and a giant opening at one end filled with sparkling energy. After the attack, Decker ordered his surviving crew to the surface of a nearby planet, but to his horror, the machine destroyed that world next. Spock theorizes the machine breaks down planets into rubble which it then consumes for fuel and adds that given its past trajectory, it is likely to have come from outside the galaxy and will continue towards the "most densely populated region of our galaxy."
Kirk theorizes that they have encountered a doomsday machine, "like the old H-bombs", a device built to destroy both sides in a war. It was intended as a bluff or deterrent, not to be actually used, but was activated nonetheless. It wiped out its builders long ago but it lives on indefinitely, fueled by the very planets it destroys.
Kirk has Decker beamed back to the Enterprise, with McCoy, for medical treatment while he and Scott remain on the derelict Constellation. On the Enterprise bridge, First Officer Spock, having been left in command, is alerted to the approach of the alien machine which generates interference that makes communication with Starfleet Command impossible. As the machine attacks, Decker comes to the bridge, and quoting Starfleet regulations he pulls rank on Spock and assumes command. He then orders a full-on attack against the machine ignoring Spock's warning that the ship's weaponry is ineffective against the doomsday machine's pure neutronium hull. As a result, the warp engines are disabled and the Enterprise becomes drawn by a tractor beam towards the machine's glowing maw.
Aboard the Constellation, Scott has managed to restore partial phaser and thrust control. At the same time, Kirk has reactivated the viewing screen, and manages to create a diversion to distract the planet-killer away from the Enterprise . As the machine veers off, Kirk orders Spock to relieve Decker of command, based on Kirk's personal authority as officially designated Captain of the Enterprise. Eventually, Decker, recognizing that the Enterprise crew would support Spock without question, yields and is escorted off the bridge by security. En route to Sickbay, however, he knocks out his guard and quickly heads to the hangar bay and steals a shuttlecraft. He then pilots it on a kamikaze course into the planet killer's maw despite the pleas of Kirk and Spock to turn back.
Lt. Sulu reports that the shuttlecraft's subsequent explosion appears to have slightly decreased the planet killer's output power. Kirk realizes Decker may have had the right idea but not enough energy to succeed. Kirk has Spock determine if the thermonuclear detonation of the Constellation impulse engines inside the planet-killer would destroy it. Spock is unsure, and he and Scott both object to Kirk's intention to remain on the Constellation to carry out the plan. Kirk has Scott rig a manual 30-second detonation timer, planning to start it and beam back to the Enterprise before detonation. Scott explains that once the timer is started, it cannot be stopped.
With everything prepared, Kirk orders the others back to the Enterprise and steers the Constellation toward the planet killer's maw. At the right moment he starts the 30-second timer and asks to be beamed out. The damaged Enterprise transporter shorts out, and Kirk is stranded on the Constellation. Scott rushes to make repairs. As the timer ticks toward zero, Science Officer Spock suggests to Chief Engineer Scott that he might complete the engineering task by trying "Inverse Phasing", Kirk issues an understated request: "Gentlemen, I suggest you beam me aboard" and watches the doomsday machine grow on the viewscreen. Scott's desperate fix succeeds and Kirk is beamed off the Constellation at the very last second. The Constellation enters the maw of the planet killer and explodes, destroying the planet killer's internal mechanisms and leaving its indestructible shell adrift, dead in space.
In the epilogue, Kirk and Spock muse about the parallels between their doomsday machine and the "doomsday machines" of late 20th century Earth, nuclear weapons. Kirk notes with irony that the Constellation's impulse engines exploded in the same way, though this time it served a constructive purpose.