The plot concerns the crew of a spaceship from the United States landing on the Moon during a period of intense Cold War tension, and the possibly disastrous consequences of their discovery there.
The story is framed as the account of an unnamed narrator telling a professor of art history about their upcoming secret mission, and why it is so critical.
The narrator tells how he and two others, Baird, the commander, and Hernandez, the engineer, flew the spaceship Benjamin Franklin to the Moon, landing just outside the crater Plato.
The narrator wants to climb down and investigate the cloud of vapor, but Baird, who does not trust him, opposes this until Hernandez intervenes.
Believing that the previous explorer must have had access to some revolutionary technology, technology that could win or start a war, the narrator reveals to the professor that the light he saw on the ledge was the same as that used in the painting Virgin of the Rocks, and that their mission is search museums and old papers to find some clue as to how Leonardo da Vinci could have walked on the Moon.