Richard Johnson is an average college student at the University of Colorado in Boulder, though he is having a particularly bad week. He just spent his last dime on food, lost his wealthy girlfriend, flunked a major midterm, and had his bike stolen. To add to his misery, he returns to his apartment after walking his landlady’s dog Khan and runs into his strange neighbor. Convinced that this bizarre man, nicknamed the Ghoul, is more than he seems, he follows the dog into the Ghoul’s apartment to find a timegate there that sends him soaring to a place that looks like Boulder without buildings.
Stumbling along after Khan towards the mountains overlooking the vast plain on which he landed, he sees a strange house at the top of the cliff. As he climbs the steep stairs leading to the house, a sudden rainstorm almost washes him away, but somehow he makes it to the front door of the house, stumbles into the empty building, trying to find Khan. Failing as night comes, he falls asleep on the floor of the top floor.
When morning comes, he is awakened by Khan licking his face. He gets up and stares out the window to find that the house overlooks a long grassy plain that runs parallel to the mountains, but on the far side of the plain is a forest of impossibly tall trees that almost look artificial.
Though Richard has no idea how to get home, Khan remembers the way, and Richard follows him back to the timegate, through which he passes back into the Ghoul’s apartment.
After trying to regroup for his classes for a day, he decides to return to the Ghoul’s apartment and try the timegate again. This time he ends up in a different location, Boulder in the year 2288, which confirms to him that it is a timegate, though he has no idea where in time he traveled with his initial journey. Far in the future, he decides.
Not sure what to do, he decides to tract the Ghoul’s activities. Stealing his ex-girlfriend’s car, he tracks the Ghoul to the Department of Genetic Therapy at the University of Colorado Medical School in Denver. There he encounters Sam Robinson, a graduate student in the department, who works near the Ghoul. He tells Richard that the Ghoul is an expert in human genetics and is working on some strange potential gene therapies. After they get to know each other, Richard decides to show him the timegate, only to discover that it isn’t working, which convinces Sam that Richard is crazy. Sam’s sister, Summer Jacklyn, is one of Richard’s teachers, and when she comes along to view the non-existent timegate, Richard is devastated.
But one day Richard arrives back at the apartment to find that his room has a flying piranha-like creature that almost kills him, though Richard manages to subdue it in the end. Richard takes the dead body to Sam’s lab, and Sam eventually examines it, to realize that it is an entirely alien creature, probably constructed artificially because it is designed to live only a few days.
When another set of creatures try to attack them along Boulder Creek, and annihilate two policemen, Sam starts to believe Richard, especially after they meet, Tao Benrobi, a man who claims he is from the twenty-fourth century. They bring Sam’s sister Summer and Benrobi back to the timegate with them. This time the gate is open and they end up on the plain with the giant forest. When they look up they see contrails drifting across the sky. The vehicles leaving them all fly eastward beyond the forest and dash to the ground on the other side of the forest. They also notice a giant army of some kind of beings far down the grassy plain, thundering towards them.
Fleeing from the army, they dash into the forest, and head for the far side.
The army follows, and they can hear its footsteps crushing through the forest. Almost killed by one the giant beasts, they escape and spend the night in one of the trees. In the morning they continue their journey towards the site where the ships had landed.
Reaching the other side of the forest, with the army of beasts behind them, they find the giant clearing littered with spaceships. With some difficulty they enter one, and Tao Benrobi takes command of it, and takes off, to the dismay of the others.
The ship flees towards the moon, where they encounter a fleet of immense battlecruisers. The ship, controlled by the computer, lands in the giant docking bay of one of the cruisers. Shortly afterwards, the cruiser begins to move. The stars turn, start to wobble and twist as the ship moves through space.
Richard has no idea where they are going, but it doesn’t matter. Soon the Ghoul and a number of the alien beasts and some small crab-like aliens appear and take them captive.
They discover that the aliens are artificial warriors and commanders genetically designed by an Alliance of alien civilizations in the heart of the Milky Way galaxy, and that they are putting together a fleet of battlecruisers and an army of warriors to fight a battle against another civilization sweeping in from a distant galaxy. This Enemy has won two previous battles against the Alliance, but this will be the last hope for the Alliance. The Ghoul tells Richard that he is genetically constructed to look like a human, and given the knowledge to create genes that will eventually wipe out mankind, because the Alliance needs Earth to center their new attack on the Enemy, so mankind has to go.
The giant starship takes them to a distant solar system where the battle is waging. Richard and his friend watch as starship after starship explodes into flames, dying in the darkness of space, but it is clear that the Enemy is winning. As the fighting wanes, almost all the Alliance ships have been destroyed. Richard and his friends escape from theirs before its demise in a small shuttle, racing across space to land on a small dark moon in order to repair their shuttle from minor damage.
Landing, they discover it is inhabited by shadowy beings who try to kill Richard and Summer as they go outside the shuttle to accomplish the repairs. Summer is overcome by them but Richard saves her, dragging her back inside to discover that Tao Benrobi has taken control of the shuttle. Richard realizes that he is one of the Enemy, genetically constructed to infiltrate mankind and decide why they are a possible threat to his civilization. He, in fact, is the one that back on Earth, constructed the alien monsters that attacked Richard and Sam.
Escaping to the damaged Alliance ship, they discover that it still functions, but that it still contains some Alliance warriors and commanders, who wish to destroy Richard and his friends.
But Richard manages to take control of the damaged battlecruiser, and program it to head for Earth, slinging around the white dwarf sun at the center of this solar system to produce the speed necessary for a jump through interstellar space. Chased by two Enemy battle cruisers, they just make it, flashing into space between the orbits of Mars and Earth. One alien ship remains, but the shuttle is so damaged it barely functions. However, Richard crashes it into the plains near Denver, using the emergency safety chambers in the ship to survive the plummet to Earth.
When Richard awakens, he follows the others through the timegate back to his time.
But the war isn’t over. Tao Benrobi has kidnapped Sam, taking him to a house in Denver where he creates his alien monsters. To rescue Sam, Richard, the Ghoul and Summer, arm themselves for the attack on Benrobi’s genetic madhouse.
The battle and the escaping monsters bring the police and National Guard, but Richard and his friends survive, though Benrobi is killed by one of his creations. The Ghoul seems lost but returns with Richard, Sam and Summer to their original house, where Khan awaits them.
They convince the Ghoul to help them reverse the genetic damage he has inflicted upon mankind, and together, using some of the Ghoul’s knowledge, they start to build mankind a better future.