Commodore Decker
William Windom

Crewman Elliot
John Copage

Crewman Washburn
Richard Compton

Crewman Russ
Tim Burns

Lt. Palmer
Elizabeth Rogers

Lt. Kyle
John Winston

Crewman Montgomery
Jerry Catron

On investigating a distress call from the U.S.S. Constellation NCC-1017, the crew of the Enterprise find only debris where there should have been a planetary system and eventually come across the badly disabled starship. Subspace interference makes communication impossible so Kirk takes a party across to the stricken vessel only to find it apparently deserted. Scotty and his engineers find the phaser banks to be exhausted and the warp engines burnt out but the ship still has a repairable impulse engine.

They find only one man on board, a distraught Commodore Matt Decker, who tells them that he had to transport his crew down from the defenceless ship to planet L374 before 'it' attacked again, and could only watch as the planet was destroyed. He gathers himself together a bit and informs the Enterprise men that the thing in question is miles long and they came across it carving up a planet with an antiproton beam.

Spock examines the ship's log, calculating that the object must have come from outside the galaxy and, if allowed to continue on its present course, will pass through the most densely populated section of their own galaxy. McCoy and Decker transport back to the Enterprise which, they shortly find, has been located by the planet-killer and which is now in pursuit. Barely able to keep ahead of the planet-killer the Enterprise briefly lowers shields to beam the other members of the crew aboard only to be fired upon by the giant 'spaceship', leaving them with damaged communications and no immediate means of retrieving Kirk and the others.

Fortunately, the planet-killer seems to lose interest in the ship and resumes its previous course towards the Rigel colonies, which has given Scotty and his team a bit of time to work on the Constellation. Spock orders the Enterprise to return to the stricken Constellation but Decker pulls rank and takes command. He is planning to fire full phasers at the planet-killer from point blank range despite Spock's warnings that the hull of the alien machine is made from practically solid neutronium and that the Enterprise is not powerful enough to disable it.

Scotty manages to restore power to the Constellation's view screen and Kirk, horrified to see his ship in pursuit of the planet-killer, orders Scotty to get him any type of propulsion power immediately. The planet-killer captures the Enterprise in a powerful tractor beam from which it cannot break loose, being freed only when an attack by the Constellation, now with some engine power restored, distracts its attention. Both ships manage to get beyond its range by attacking alternately, at which point Kirk contacts the Enterprise and orders Spock to take command.

Decker is sent to sickbay but overpowers the security detail and steals one of the shuttlecraft which, driven insane by guilt and the need for revenge, he flies straight into the maw of the planet-killer. The Enterprise sensors register a slight drop in power from the planet-killer as the shuttlecraft's engines explode which gives Kirk an idea. Beaming everybody except himself and Scotty back to the Enterprise, the Constellation's engines are wired to explode on a thirty second time delay.

Scotty beams back, leaving Kirk alone on board to steer the Constellation towards the mouth of the planet-killer. Tense moments follow as the Enterprise transporters develop a fault but they are repaired in the nick of time to beam Kirk out with only seconds to spare. The planet-killer 'swallows' the Constellation whose engines detonate moments later, the explosion killing all signs of energy from the alien machine. Spock cheers them up later with the thought that if this was a 'doomsday' machine built as an ultimate deterrent by some distant civilisation, then it is quite likely that an opposing civilisation would have had something similar as well.