The book collects a series of short stories by Niven, including "The Flight of the Horse", in which he introduced the Temporal Research Institute. The premise of the novel is that the old Secretary-General has recently died; his heir is more interested in extraterrestrials than the Earth beasts who went extinct due to excess pollution. This resulted in the Temporal Institute attempting to invent space flight, and several of its agents visiting Mars.
The starting temporal point is the year "+1108 Atomic Era," corresponding to 3053 AD in the Gregorian Calendar. The main character is Agent Hanville Svetz, a brave and resourceful man. Neither he nor any of his colleagues ever realize that all his expeditions have actually been to parallel universes, and the animals he retrieves are all creatures from fiction, such as Moby-Dick, and from mythology, such as Quetzalcoatl. The idea that he may be traveling into realms of fantasy does occur to Svetz a couple of times, but the notion is rejected by others.
When the expedition arrives on Mars, the year is "-550 Atomic Era," corresponding to 1395 AD/CE.