An alien artifact has been discovered on the dark side of the Moon--a labyrinth of some kind, which kills everyone who enters it and fails to follow strict (and unknown) rules.
A brilliant scientist, Doctor Edward Hawks, has invented a matter transmitter--a kind of teleportation device. With the discovery of the lunar artifact, the military funds further development of his invention, hoping to use it to solve the alien labyrinth before the Russians discover its existence.
The matter transmitter does not actually move people from place to place, however: it scans them, destroying them in the process, and constructs a perfect duplicate at the destination.
There is one further wrinkle: the labyrinth does not permit information to be conveyed beyond its gateway--not without killing the person communicating it. How, then, can any progress be made?
The matter transmitter is capable of making as many copies of a person as desired, so it is possible to send a man to the moon to try the labyrinth, to die, while leaving a duplicate--a second duplicate--on Earth, to live.
It is discovered that the two duplicates, being of one mind, share experiences across the void--though they quickly diverge enough that they cease to be in communication. Through careful preparation, it is possible to extend the length of this contact, so that the duplicate on Earth effectively experiences the labyrinth without dying in the process. However, the memory of dying is enough to drive a man insane, so even this is not quite enough to crack the labyrinth.
The final piece of the puzzle is an unusual man: Al Barker, a thrill-seeker who is said to love death. Hawks reasons that such a man may not be so strongly affected by the experience of dying as to be rendered useless--and he is correct.
Day by day--life by life--Barker works his way through the labyrinth. At last, when he is sure that his next attempt will succeed, Hawks joins him and they complete it together. Coming out the other side of the labyrinth, they find... nothing!
Why should they have expected anything else? Who knows for what reason the labyrinth was created. Surely it wasn't with humans in mind. So, when they have successfully navigated the labyrinth, they simply exit it without dying and report their success to the military observers. Knowing how to survive it, now, technicians will come to disassemble it and learn from it.
The lunar duplicate of Hawks walks out into the lunar wastes to die out among the rocks, so that the life he left behind can be lived by his Earthly counterpart. The duplicate of Barker chooses to live on the Moon in hopes of one day returning to Earth, when a transmitter has been built on the Moon (where there is presently only a receiver).
Only a fairly small part of this story is actually devoted to the problem of navigating the labyrinth. The bulk of the story is concerned with developing four characters: Hawks; Barker; Vincent Connington, the personnel director at the company that employs Hawks; and Claire Pack, Barker's lover. Each has a unique way of understanding and dealing with the world: Hawks attempts to understand the world; Barker tests himself against it; Connington manipulates people to elevate himself; Claire manipulates men through sex apparently out of some kind of self-hatred.
The major theme of the novel is death. The labyrinth kills everyone who enters it--but still people keep at it. The matter transmitter kills everyone who uses it, perhaps creating indistinguishable duplicates. Hawks' assistant (and friend of ten years) Sam Latourette is dying of cancer and hopes that the transporter can help him avoid death. Barker courts death to prove his worth. Hawks must deal with the moral implications of killing people--many people--in a scientific endeavour of unknown value. And the military observers on the moon face another kind of death--the loss of their lives on Earth, which are being lived by other versions of themselves, and to which they can probably never return.
The final encounter with the labyrinth itself is not so well done. Throughout the book, it is said that the labyrinth operates on rules with no apparently logical purpose:
However, when Hawks and Barker are navigating the labyrinth, all the obstacles described are basically just physical: jump over this, duck under that, strike this wall. It's a bit disappointing.
It is a little disappointing that nothing ultimately comes of successfully navigating the labyrinth. Or better, perhaps, to say that success and failure are simply human constructs. The thing is a metaphor for life, and the idea of life and death is only meaningful because we take note of it; the universe no more cares for our lives than it cares whether we can get through an alien labyrinth. So it is very thematically appropriate that nothing comes of it, but it's still somehow unsatisfying.
I found Hawks an interesting character, but Barker was infuriating, and the other two somewhere between uninteresting and insufficiently developed. Much as I like to see SF focus on things other than bug-eyed monsters, I think this story, short as it is, could have been trimmed down a bit without losing anything important.