The story deals with the discovery of an artifact on Earth's Moon left behind eons ago by ancient aliens. The object is made of a polished mineral and tetrahedral in shape, and is surrounded by a spherical forcefield. The first-person narrator speculates at one point that the mysterious aliens who left this structure on the Moon may have used mechanisms belonging "to a technology that lies beyond our horizons, perhaps to the technology of para-physical forces."
For millennia (evidenced by dust buildup around its forcefield) the artifact has transmitted signals into deep space, but it ceases to transmit when the astronauts who discover it breach the forcefield. The narrator hypothesises that this "sentinel" was left on the moon as a "warning beacon" for the possible intelligent and spacefaring life that might develop on Earth.
This quotation illustrates the idea, and its ramifications:
It was only a matter of time before we found the pyramid and forced it open. Now its signals have ceased, and those whose duty it is will be turning their minds upon Earth. Perhaps they wish to help our infant civilization. But they must be very, very old, and the old are often insanely jealous of the young.
In the movie 2001: A Space Odyssey, the operation of the sentinel is reversed. It is the energy of the sun, falling for the first time on the uncovered artifact, that triggers the signal that creatures from the Earth had taken the first step into space.
