While engaged in a surveying mission light years from Federation territory, the starship Enterprise receives word that the Organians — the advanced beings who enforce peace between the Federation and the Klingon Empire — have suddenly vanished. As they begin the months-long journey back through Klingon space to investigate, Scotty develops a new version of the transporter, one designed to teleport a person across the galaxy instantaneously. When it is used to send Spock to the Organian homeworld, however, the transport fails, producing two indistinguishable Spocks. Captain Kirk is now faced with the task to deciding which one is the true Spock, and which is the reversed duplicate of his friend who must be destroyed.
Such is the premise of James Blish's novel, which is something of a historical artifact. Originally published in 1970, it is the very first original Star Trek novel written for adults, the progenitor of the shelves of novels, novellas, and short story collections that have been published since. In this respect Blish was blazing a trail followed by everyone since, which makes reading it from today's vantage point an interesting experience. Longtime fans will find more than a few idiosyncracies and anachronisms in its pages, while the story's resolution is so overblown as to leave the reader wondering whether Blish seriously believed that it would hold up. Such reactions, though, point as well to the underlying pleasure of the book, which bears virtually none of the weight of the overstuffed franchise and still holds value as a result.