On the planet Strada there exists a technologically advanced civilization, one that has conquered space travel and which has colonized scores of other planets. Society is orderly, efficient and maintains a single ideology. The government has evolved into two separate camps of governance: The League, which supervises the administrative affairs of the society, and The Center, which concerns itself with science. They are the two equally-powerful branches of Stradian society, and they have no love for each other. In fact, something akin to an unspoken Mutually Assured Destruction agreement exists between the two bodies. Agents from either branch routinely go undercover and assassinate members of the other branch, in furtherance of their own agenda. It is an underground, piecemeal conflict that goes on with the unspoken knowledge of each other's actions. Any attack overtly carried out would result in open warfare and, in all probability, the end of their civilization. It is this assurance that keeps each side honest and allows for the orderly running of society.
Amro, an agent for The Center, has been assigned a new mission, one to take place on another world. He studies and learns the language, then the history of this strange place he has never heard of. Outside of his unfamiliarity with this planet, it seems like just another mission. He's been doing this for five years and this will be his fifteenth undercover assignment.
There is a reason Amro has never heard of this new place: it is farther away than any spaceship could travel. It has been discovered by the scientists of The Center, a twin world in a kind of different dimension, reached not by spacecraft but by passing through a portal that The Center has learned how to create. The Center is going to use this newly discovered world, unknown to the members of The League, as a staging area to launch a final attack for complete dominance of their empire. The new world is Earth.
In "Shadow On the Sand" John D MacDonald wrote a 30,000 word novella, published in the October 1950 issue of Thrilling Wonder Stories. Full of long, descriptive detail of Stradian government and society, it is a wordy piece that would been a much more endurable read with 10,000 less words. JDM's great burst of science fiction writing was drawing to a close and he would only write eleven more s-f stories in the 1950's (out of a total of 55). What does make "Shadow On the Sand" an interesting story (at least for me) are the characters and scenes that take place on Earth. Here MacDonald was in more familiar territory and his confidence shows.
In order for Amro and his fellow Stradian agents to infiltrate Earth, they need to find and replace individuals. Three Earthlings are kidnapped and brought to Strada, their memories harvested and their outer skin removed. That skin and memory is then grafted onto the agents, who "become" that person. The people they find are a married couple and a male houseguest, enjoying an extended stay in a remote beach house near Harlingen, Texas. Jerry, the husband, is a chemical engineer who has been injured on the job and has been given a six month leave of absence to recover. He's also a cuckold. The male houseguest, strong, attractive Quinn French is having an affair with Jerry's wife, Fran. Jerry suspects something but is too much of a wimp to do anything about it. In an effort to cover their tracks, they've decided to have Quinn invite a female friend to stay for awhile, someone who he hasn't seen in years, but who can throw Jerry off the scent of their adulterous relationship. By the time Martha Kaynan arrives, however, Jerry, Fran and Quinn have all been replaced by Amro and two other Stradian agents. Martha's gradual unease and eventual discovery of the charade is the real meat of the story.
At times it becomes obvious to the reader that MacDonald was being paid by the word. The explanation of the portal is especially mind-numbing:
"We have made the basic and very important discovery of a twin planetary system corresponding to our own, separated from us only by a symbolic logician's definition of reality. This is not a completely physical and technical phenomenon. It is a philosophical phenomenon.
"In simplest terms the formula can be expressed this way: The twin world exists because any definitive explanation of reality presupposes alternate realities. Thus the doorway was achieved by the creation of unreality. Call it negative matter if you will. A sphere where there is no reality must, through the application of the basic formula, be a bridge between realities. The bridge had been achieved but there is much that we don't know."
Martha Kaynan is the real hero of the story, and is the typical plucky MacDonald female: "She was a small girl with brown hair that sometimes glinted red in the sunlight. Her eyes were a soft and smoky aqua and her mouth had a childish look. A careless observer might think her a quite low-pressure little girl, possibly a bit dull. But the careless observer missed the lift of her chin, the directness of the eyes, the squared shoulders, the determined walk." And although Martha views Quinn as "a definitely unwholesome type," the invitation comes at a time when things in her life hadn't been going too well. Of course, she never does get reacquainted with Quinn, she encounters a disguised Amro instead. Their eventual relationship is the engine that drives the story along.
I'll confess to being someone who doesn't really read much science fiction. The only books in my library that could be called s-f are the works of Ray Bradbury and several old, collectable volumes of Charles Beaumont stories, and these can't really be classified exclusively as science fiction. A well told, well-written story is a joy in any genre, however, and "Shadow On the Sand," if you have the patience, contains a good story buried underneath an awful lot of words.
I found a reference to the story online, indicating that "Shadow On the Sand" may be the first-ever story containing the idea of alternate reality, or an alternate world. If this is so, hooray for MacDonald, but reading the citations makes my head hurt. If you like you can try for yourself here.