Roadside Picnic is set in the aftermath of an extraterrestrial event called the Visitation that took place in several locations around the Earth, simultaneously, over a two-day period. Neither the Visitors themselves nor their means of arrival or departure were ever seen by the local populations who lived inside the relatively small areas, each a few square kilometers, of the six Visitation Zones. The zones exhibit strange and dangerous phenomena not understood by humans, and contain artifacts with inexplicable properties. The title of the novel derives from an analogy proposed by the character Dr. Valentine Pilman, who compares the Visitation to a picnic.