The Enterprise crew is carrying out a protocol call upon the planet Nraka, which is celebrating its 10,000th year of recorded history. Captain Kirk and Mr. Spock meet with Councilor Dvor to offer their official congratulations, and Spock makes a traditional cultural toast to the planet's most famous historical figure, Zotar. Dvor is pleased that the Federation has learned so much of his culture. He leads Kirk, Spock and Scotty to his palace balcony, where they watch a fireworks celebration in honor of the occasion. Scotty is accompanied by Miss Rava, the daughter of the supreme councilor. (And both seem to enjoy one another's company quite well, judging from how closely they hold each other!) Once the fireworks are finished, Dvor directs the attention of everyone to the East, where a huge bright spot rises in the sky, revealing the image of a man wearing armor. "Look, Mr. Spock" Kirk exclaims, "it's a giant movie screen!". "Exactly captain!" Spock replies. "Lasers of invisible light activated by the chemical particles! That is an actor made up as Zotar!".
A large black spot appears in the middle of the stellar show, and begins growing larger. Nrakan Councilor Sja-Yo is alarmed, and he takes Kirk and Spock to a nearby observatory. He quickly brings up an image of the object - a huge spherical space ship. "It is a foreign space vessel" says Sja-Yo, "larger than anything we have ever dreamed of! Several miles in diameter by my estimate!".
"Hear me well!" the armored figure continues. "Once my people, the Sanoora, dwelt upon your world, climbed its mountains, fished its streams, 500,000 years ago! Now we have returned, and it is you who must go!". The Nrakans are stunned by this announcement, and the crowds become frightened. "You have many questions, of course! But we have prepared a history of my people to answer them!". As Inoduku narrates, images of the past flicker on view screens around the planet. "Once we were a great and powerful people" Inoduku says. "Our cities reached to the very clouds themselves!". Inoduku tells how, on a day that became known as "The Black 23rd", scientists discovered that a rogue star would pass nearby the planet in 342 years, destroying all life. From that day forward, their entire civilization thought of nothing else but "The Great Expedition" from their world.
The transmission shows how huge ships were constructed, each six square miles, to carry the inhabitants away from the planet. Even so, many had to be left behind. Violent criminals, incurably sick, and highly disturbed people were chosen to sit outside and watch as the great ships took to the stars. Then the planet was ravaged by the rogue star. "Since all radioactivity would have cleared in 500,000 years, we chose an orbit that would return us here in that time" continues Inoduku, "But we are weary of wandering! We reclaim our home! We will give you our ships and you may find another home! But we want our sacred planet back! You have one day to consult and decide!". At that moment, a huge blast from the orbiting space ship completely destroys a ancient, abandoned city on the surface of the planet. "We have the power to do that to all of your cities in a matter of minutes!" finishes Inoduku. "Think well upon your answer!".
The Nrakans cannot believe that their race sprung into existence in the 500,000 years since the Sanoora left. Mr. Spock speculates that the people left behind when the rogue star came were able to equip a giant cave, and survive the star's passing. Later, when the star had passed, the people emerged from the caves and had children, who were immune to the remaining radiation.
Back at the observatory, Councilor Dvor is angered as Mr. Spock explains that due to the Federation policy of neutrality, they will not assist his planet in fighting the Sanoora. The Enterprise contacts the landing party by communicator. Mr. Sulu reports that an oversized ship of unknown design is approaching. "I know what it is, Lt. Sulu" Kirk answers. "Use only defensive tactics to avoid contact!". Just then, the Sanooran ship fires a blast that narrowly misses the Enterprise, and Inoduku quickly appears again on a view screen in front of Kirk and the Nrakans.
After the attack and threat, Mr. Spock's attitude of non-interference quickly changes. "General" he says, "you will place a technical crew at my command immediately! I will adapt your present weapons for full penetration!". Later, as the Sanooran ship chases the Enterprise, a fleet of newly re-armed Nrakan space craft attack it. Now able to penetrate the huge ship's shields, they damage it severely. Soon, the giant ship turns and begins to retreat back into deep space.
"It would seem so, Captain!" Spock replies. "And now they are doomed to continue wandering space, a 'world' without a planet to live on!". "Was there any choice, Mr. Spock?" Kirk wonders aloud, "Could we have found a better way out? I suppose we'll never know!".