Ram Odin is the highly trained pilot of a starship carrying sleeping human colonists away from Earth and toward the planet of Garden. Since Garden is so far away from Earth, Ram hopes to leap through a fold in space-time, and cut 900 years off their journey. Ram's only company is an "expendable," a machine that simulates human life so well that it is capable of making decisions and provides him with company throughout the journey. If Ram's starship makes the jump, other ships from Earth will follow with valuable supplies and other colonists.
The jump is made successfully, but with an unintended consequence. Eighteen other copies of Ram's ship are created, complete with copies of the colonists, Ram and the expendable. But they are not created in the present. All 19 ships find themselves 11,191 years in the past. Ram orders the expendables on the other ships to kill the other Rams. They do.
To maximize the chance of human life developing successfully on Garden and minimize the problems that could arise by having multiple human copies in the same place at the same time with the same DNA, Ram and the expendables agree to divide Garden into 19 different areas separated by an impassable wall. When the humans learn to breach the wall, they will once again be in charge of the expendables and privy to any information that the expendables will choose to keep from them.
Rigg is a poor kid who spends most of his time in the high country above Stashi Falls. The calendar that began at 11,191 reached year zero when he was 3. Now he is 13, and he and his father travel together through the wilderness, trapping animals to sell their pelts in the village of Fall Ford. Despite their humble trade, Father is insistent that Rigg's education include the study of mathematics, languages, astronomy and other subjects that Rigg cannot imagine ever being useful for a life in the woods. But Rigg has one skill that does not need to be taught. He can see the paths that every living creature, animal or human, leaves as it travels through the world — except one. He cannot see Father's path. Father works with him to hone this inborn ability and warns him never to tell anyone else.
But then Father dies. Rigg never sees his broken body, since Father warns him away. Father's torso has been pierced by the branches of a falling tree, and he doesn't want Rigg to have the grisly image stored in his memory. With his dying breaths, he tells Rigg to take the furs to Fall Ford and sell them. Then he is to ask Nox, the woman who runs the rooming house, how to find his sister. Rigg is shocked to learn that both his sister and mother are still alive. But Father doesn't respond to his frantic questions. After waiting for a suitable length of time, Rigg tells Father that he loves him. Then he leaves without looking back.
The most perilous part of the trek back to town is crossing above Stashi Falls. There is no bridge, only a series of boulders. Rigg must leap from one to the next, carrying a heavy load of furs on his back. But when he is halfway across, he sees a small boy attempting to make the crossing from the other side. Rigg can see that the route the boy has chosen is one of the most dangerous — many of the paths lead over the edge of the falls. When the boy slips, Rigg abandons his furs in a desperate effort to save him. But just as he is about to grab the boy's hand, time slows down and the paths morph into real, solid people. One man in particular blocks him from reaching the boy. When the man latches onto Rigg's arm, Rigg peels his fingers away, but the boy plummets over the falls anyway, time speeds up and the man disappears.
Unfortunately, Umbo, the boy's brother, was watching from the bank. Umbo believes Rigg killed his brother and races back to town. By the time Rigg arrives, Umbo has already shared his version of the events, and the men of the town are determined to kill Rigg. He hides in Nox's rooming house until she convinces the townspeople that Rigg is innocent. She also tells Rigg that his mother and sister are in the city of Aressa Sessamo. She gives Rigg a bag of 19 jewels and a letter of credit.
Rigg leaves Fall Ford alone, but Umbo catches up with him. His father kicked him out, and he wants to join Rigg on his journey. Rigg agrees but is confused that he and Umbo seem to have differing memories about the legend of a certain Wandering Saint. Rigg realizes that the Wandering Saint was the man who stopped him from rescuing Umbo's brother and that by interfering with the past, he has changed everyone's memory except his own. He learns that Umbo also has a special skill — he can slow down time. By working together, Rigg and Umbo can use their combined talents to travel into the past. When they reach a tavern in the town of Leaky's Landing, they are befriended by the innkeeper, Loaf, and his wife, Leaky. But when Leaky does their laundry, she discovers the jewels Rigg had sewn into his clothing. Loaf, a retired soldier, offers to escort the boys to O — the nearest city that has a bank. This will allow Rigg to liquidate one of the jewels for enough cash to travel the rest of the way to Aressa Sessamo.
In O, Rigg learns that Father's education has prepared him for the civilized world. He negotiates skillfully with the banker, exchanging a jewel for the needed funds. On the day they plan to leave O, the threesome decides to visit the tower of O. They realize that the huge globe inside the tower is actually a map of the world (maps are illegal) and that the wall enclosing their country, or wallfold, also encloses 18 other wallfolds on the planet of Garden. While leaving the tower, Rigg sees a future version of Umbo warning him to tell Loaf to hide the jewels. Loaf does — just in time.
The three of them are arrested by General Citizen and forced aboard a riverboat heading to Aressa Sessamo. Apparently, the jewel Rigg had sold was a well-known ancient jewel that had been lost centuries before, and the letter of credit named him as the lost prince of the deposed royal family. Loaf and Umbo manage to escape while Rigg is being interrogated. Loaf and Umbo head back to Leaky's Landing. Rigg is taken to Aressa Sessamo. General Citizen seems pleasant enough and tells Rigg that the family resemblance is unmistakable — he is certainly the lost prince. Nevertheless, Rigg lands in solitary confinement for the rest of the journey after pre-emptively rescuing himself from an assassination attempt.
In Aressa Sessamo, Rigg meets his mother, Hagia Sessamin. Rigg doesn't know how she feels about his sudden return. The Sessamid Empire was matriarchal and has a history of killing male heirs. Others may see him as a threat to the newly formed People's Republic and want him dead as well. He uses all of the rhetoric and political savvy that Father taught him and paints himself as a country boy who thinks he's no better than the lowest junior kitchen helper. But he is still astute enough to ask probing questions that make it uncomfortable for people in power. The common people love him, but Rigg knows he walks a fine line between life and death.
Rigg also finds and befriends his sister, Param. Like Rigg, she has a special talent. Param can become invisible by skipping forward in increments of time. Rigg also learns that his real father, Knosso Sissamik, was a scholar. He died while trying to pass through the wall. Rigg gains access to the library and learns everything he can about his father's research. He is accompanied to and from the library by Olivenko, a young soldier who loved and respected Knosso. During the course of his studies, Rigg realizes that not only is the wall man-made, but every human being and many of the animals and plants on the planet of Garden are not native to the planet. They originated somewhere else.
In the meantime, Umbo learns how to project himself backward in time to give warnings. He and Loaf retrieve the jewels in O and travel to Aressa Sessamo to meet Rigg.
Since the royal family was forbidden to own any property when their rule was overthrown, Rigg, his mother and his sister are boarding with a wealthy man named Flacommo. When Flacommo is killed one morning, Rigg knows he must escape with his family. But his mother is not surprised by the attack, which is led by General Citizen. She encourages Rigg and Param to give themselves up. She plans to marry the general and create a new dynasty. Rigg and Param know they will be killed so the general's children can inherit the throne.
Using their combined skills, Rigg, Param and Umbo flee the city with Loaf and Olivenko. They are no longer safe anywhere in the Republic, so they plan to escape through the wall. Although the wall is transparent and not solid — animals and plants can pass through unharmed — it exerts a powerful force over humans. Even being close to the wall can produce feelings of extreme anxiety and despair. Humans who have attempted to pass through have been driven insane before reaching the other side.
With General Citizen's men closing in, Umbo sends Rigg, Loaf and Olivenko back to a point in time before the wall existed. They barely make it to the other side alive, as the time Umbo chose was the day that the 19 starships from Earth were landing on Garden, and he is forced to bring them back to the present before Rigg has fully passed through. Loaf and Olivenko, who have a natural resistance to the wall's power, rescue Rigg. Just as they are about to be captured, Umbo and Param also travel through the wall. They combine their skills to arrive safely before Rigg and the others. But a surprise awaits them all.
It is Father. Rigg is shocked to see that the man he called Father is still alive. But Father doesn't recognize Rigg. Instead, he calls himself an expendable and says that both he and the man Rigg called Father are machines — that is why Rigg couldn't see Father's path. Now that they have learned to breach the wall, the expendable must answer the questions of Rigg and his band of travelers. The jewels Rigg carries have the power to shut off all of the walls on Garden. The jewel Rigg tried to sell and is now in General Citizen's possession will allow him to shut off the wall around his own wallfold — also called the Ramfold. He also learns that travelers from Earth are expected shortly — travelers who expect Garden to be uninhabited and ready to be colonized, not populated with cultures that are 11,000 years old. And there is no telling what those space-travelers might do.