The story "If I Forget Thee, O Earth..." begins by introducing Marvin, a ten-year-old boy. Marvin and his father hurry through a spacious building, which features a greenhouse and an observatory. They then enter an airlock chamber, where they board a scout car and drive outside. Until now, Marvin has only glimpsed the outdoors through photographs and television. At this stage, Clarke hasn't disclosed their exact location, but he begins dropping hints that they are not on Earth once Marvin and his father exit the airlock. The sun moves across a pitch-black sky, a phenomenon impossible on Earth due to its atmosphere. On Earth, a black sky indicates the sun has set, usually followed by the moon's appearance. Additionally, Marvin recalls reading the classic rhyme "Twinkle, twinkle, little star" in one of his father’s books, and he's taken aback to see that the stars do not twinkle. When stars are viewed with the naked eye from within Earth's atmosphere, the atmospheric turbulence causes them to appear to twinkle, a phenomenon known as scintillation. The lack of this effect is another clue from Clarke that Marvin and his father are not on Earth.

They travel at a speed of one hundred miles per hour in their car, which is equipped with balloon tires, unlike most cars on Earth that have rubber tires. They pass a mine and descend the steep edge of the plateau where their colony is located. Crossing a shadow line, the sun vanishes, plunging them into darkness. Hours later, after navigating through mountains and valleys, they pass the wreckage of a crashed rocket, further indicating they are not on Earth. After many more hours, they reach the end of the mountain range and descend into a valley. With the sun obscured from the valley, Marvin is surprised to find it lit by an unusual white light. Marvin and his father sit silently for several minutes as Marvin adjusts his eyes to the glare of the planet emitting the bright white light.

At this moment, Marvin can make out through the hazy atmosphere the shapes of continents and polar ice caps, identifying the planet as Earth. He laments that he has never experienced Earth's varied climate and wonders why, given that Earth appears so serene. However, as his eyes adjust further, he notices that the shadowed half of Earth, which should be completely dark, is shining with a radioactive glow—evidence of an atomic war that has ravaged Earth. Clarke then reveals that Marvin and his father are observing from a quarter of a million miles away, indicating that they—and their colony—are on the moon, a conclusion the reader might have already inferred from earlier clues.

Marvin’s father recounts the tale of Earth's destruction to him, a story Marvin has heard before but only now begins to fully comprehend. He explains how the humans at this lunar outpost, likely the last vestige of human civilization, struggled to survive. Deprived of regular supplies from Earth, they had to adapt to the moon's harsh environment, their new home in exile. However, their greatest battle was not against the environment. Marvin’s father shares that the biggest challenge was preserving their will to survive, knowing that none of them, including Marvin, would ever return to Earth. It will take centuries for Earth to rid itself of radioactivity, meaning only Marvin’s distant descendants might one day return. Marvin understands that he, too, will one day bring his own child to this place, as his father has, to pass on the tradition and keep alive the dream of eventually returning home. On the way back, Marvin is deeply affected by the sight of the home he will never visit, choosing not to look at Earth again.