The story tells of a shop owner, named Mr. Cave, who finds a strange crystal egg that serves as a window into the planet Mars.
The story was written the same year in which Wells was serializing The War of the Worlds in Pearson's Magazine, a year before it was published as a novel. Because of the vaguely similar descriptions of the Martians and their machines, "The Crystal Egg" is often considered a precursor to The War of the Worlds, though there is no clear foreshadowing of the events that transpire in the novel.
In 1952, "The Crystal Egg" was made into an episode of the science fiction anthology television series Tales of Tomorrow. The episode was a half hour in length and told in first person narration. The episode is vague as to whether it is set in the 1890s when Wells published his story or in the 1950s when the show was being broadcast. However since he is recording his story on a phonograph record it is more than likely that it is supposed to be the 1950s.
The story was adapted as part of the second episode of the 2001 miniseries, The Infinite Worlds of H. G. Wells. Wells himself appeared as a character, who is inspired to write The War of the Worlds from the events. In the show, the crystal egg comes to Earth as a meteorite and serves as a two-way teleportation gate as well. Cave is divided into two characters, a married couple (William and Rosa); a Martian scientist takes Rosa back to Mars through the egg, before rendering it inoperable. British Intelligence later comes to acquire the egg.