Three astronomers, who have been working on the Moon, Mercury and the asteroid Ceres, meet for the first time in ten years at a convention on Earth.

In the course of his investigation, a photographic record of a research paper by Villiers describing his theory is discovered on a windowsill of the room, but is found to have been ruined through exposure to sunlight.

The key lies in the idea (at the time of writing believed to be true) that Mercury has one face always pointing away from the Sun.

The guilty party had hidden the film in what he thought was a safe place because he subconsciously expected the night to last forever.

Asimov noted that in his other Wendell Urth story, "The Singing Bell", travel by teleportation was regarded as routine.