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Following a trail of destroyed solar systems, the 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 claims that a machine was slicing up planets with "pure" antiproton.

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 estimate if the overloading and subsequent thermonuclear detonation of the Constellation impulse engines inside the planet-killer might 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 activated, 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 activates the switch that begins the engine overload and asks to be beamed out. The damaged Enterprise transporter shorts out, and Kirk is now 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" as he watches the doomsday machine grow larger on the viewscreen. Scott's desperate fix succeeds and Kirk is beamed off the Constellation at the very last moment. 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.[1]

Errors and ExplanationsEdit

The Nitpicker's Guide for Classic TrekkersEdit

Plot OversightsEdit

  1. Finding the Constellation deserted, Scott suggests that he, Kirk, and McCoy go to Auxiliary Control to play back the duplicate copy of the captain's logs. The next shot shows Kirk almost walking by a room, noticing Decker, stopping, and entering the room. The room looks suspiciously like Auxiliary Control, and Scott accesses the duplicate captain's logs a few moments after arriving. If this is Auxiliary Control, why did Kirk almost walk past it? It may be located in a different location to it’s counterpart on Enterprise, and probably looks different due to the extensive internal damage.
  2. The inability of McCoy to certify Decker as unfit for command seems a bit too convenient in this episode. Spock states that for McCoy to do so, the doctor must administer a battery of tests. “Court Martial" indicates that McCoy is a specialist in space psychology. Decker is obviously distraught, in shock, and behaving irrationally. Does it really make sense that Starfleet would require a complete examination during a crisis to relieve a commander from duty? Wouldn't the visual assessment of a trained medical professional—especially one with a background in psychology—be enough? Decker would most likely refuse to accept McCoy's assessment without undergoing an examination, and would very likely use his seniority to refuse to submit to such an examination anyway.
  3. Decker doesn't make a great showing in the episode. It sounds like he realized that the doomsday machine was chopping up the planets in the system, yet he beamed his people down to the next one in line for destruction. Also, prior to the first attack on the Enterprise, Decker doesn't bother to give Spock any information concerning the planet killer's weapon range. This leaves the Vulcan unprepared for the machine’s onslaught. Decker’s refusal to share information could be due to shock at the loss of his crew.
  4. It is truly amazing that the designers of the planet killer went to all the trouble of constructing a “neutronium" hull and never accounted for the possibility that someone might try to fly a bomb down the mouth of their invention. The planet killer does have a powerful cutting tool. Doesn't it seem likely that it would use this tool to dissect any possible threat before gulping it down? The dissection function probably only works on planets that can be used as fuel.
  5. Relieved of command by Spock, Decker hops a shuttle. As the bay doors slowly grind open, Sulu’s panel light ups and he informs Spock that the bay doors are ‘opening. Of course, Spock says to shut them, and Sulu says he can’t because it's too late. There is no mention that Decker overrode the door control or jammed any systems. Evidently he strolled into the shuttle, hit the remote control, and the doors started opening. Wouldn’t the door sensors tire off a message as soon as the process began? The doors sure don't look like they open very fast. Why isn't there time to stop them’? They probably need to open all the way before they can be closed.

Nit CentralEdit

  1. John A. Lang on Friday, May 06, 2005 - 7:50 pm: When Scotty plays back the Commodore's log it starts out by saying: "Captain's log...." Er...shouldn't that be "COMMODORE'S Log"? ccabe on Saturday, May 07, 2005 - 12:48 pm: He's captain of the ship, regardless of rank. If Ensign Chekov made an entry it would still be a captain's log. John A. Lang on Sunday, May 08, 2005 - 7:36 am: Not necessarily. In Metamorphosis & Friday's Child, Scotty said, "SHIP'S log", not "Captain's log". Nove Rockhoomer on Monday, May 09, 2005 - 9:04 am: Commodore would be Decker's rank, but captain would be his position relative to the ship. At least that's what I would assume.
  2. Snick (Snick) on Friday, June 01, 2012 - 8:32 pm: Are the shields so close to the hull that the debris asteroids can come perilously near to the Enterprise without being deflected? It looked like one missed the nacelles by only a few dozen yards. Francois Lacombe (Franc0is) on Sunday, June 03, 2012 - 8:58 am: In The Wrath of Khan and The Voyage Home, the display panels showing shield status do show the shields hugging the ship hulls very closely.


  1. The Doomsday Machine at Wikipedia

The Original Series Season 2
Catspaw I Metamorphosis I Friday's Child I Who Mourns for Adonais?I Amok Time I The Doomsday Machine I Wolf in the Fold I The Changeling I The Apple I Mirror, Mirror I The Deadly Years I I, Mudd I The Trouble with Tribbles I Bread and Circuses I Journey to Babel I A Private Little War I The Gamesters of Triskelion I Obsession I The Immunity Syndrome I A Piece of the Action I By Any Other Name I Return to Tomorrow I Patterns of Force I The Ultimate Computer I The Omega Glory I Assignment: Earth