The Guardians explain that they are almost immortal, that they have been guarding “the time machine” for so many eons that Kirk and his party are the first visitors they have had since two hundred thousand years before dinosaurs walked the Earth. Kirk expresses astonishment at the concept of a “time machine,” that he had always thought it was the fable of lab technicians when they had had too much pure grain to drink. The Guardians nod their heads in the direction of a shimmering pillar of light, set between the gray-silver rocks. The time machine. Built on lines of creation that mortal Man will not discover for a hundred hundred times the span of years he has been in existence. Created out of pure matter, and harnessed to this world where the passage of time and space meet just so. The pillar of light rises up and disappears. Into that fire of forever lies the passage to the past.

Kirk advises one of his techs to bind Beckwith in a straitjacket of force-fields, till they have spoken more with these ancients, and made some decision as to whether the sentence can be carried out here, or they must go to space again, to find a truly uninhabited world.

Beckwith is taken away and Kirk asks the Guardians if they would consider it an imposition to tell them more about the time machine. They smile a little wearily, and say they would be pleasured to do so. “We want to know,” Kirk says gently, and they answer: We have nothing to do but desire to show you. Uncounted millennia they have stood here, silent, and to exercise their craft is their delight.

They explain how the time machine works, and then they offer to show Kirk the past. He asks if they can show him the past of any world, and they say yes. Kirk asks to see the past of Old Earth. They show him the time of the mastodons, the time of the clipper ships, the time of the Depression, 1930. And the men of Earth marvel.

Is it possible to go back, they ask.

Yes, it is possible to go back, but not wise. Man and non-Man must live in their present or in their future. But never in their past, save to learn lessons from it. If passage back is effected, the voyager may add a new factor to the past, and thus change everything from that point to the present all through the universe. It is dangerous.