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.

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, as the Martian effort to observe and study humanity remotely might indicate their preparation for an eventual invasion.. Mr. Cave has an antique shop in Seven Dials, a district in the West End of London which, at the time the story was written, was a poor area.

He is a friend of Mr. Jacoby Wace, who is Assistant Demonstrator of Anatomy at a nearby teaching hospital and is interested in unusual characters like Mr. Cave.

Studying it closely, "it gave him the impression that the object had for a moment opened to him the view of a wide and spacious and strange country."

Mr. Wace is unable to see the view inside the egg as clearly as Mr. Cave, whose fatigued mental state seems to enhance his perception of the vision seen within it.