Dr. Lawrence Angelo works for Virtual Space Industries. His part in "Project 5" involves increasing the intelligence of chimpanzees using drugs and virtual reality. One of the experiment's chimps escapes using the warfare technology he was being trained to use. Angelo is revealed as generally a pacifist, who would much rather explore the intelligence-enhancing potential of his research without having to apply it for military purposes.
Jobe Smith, the "lawnmower man" of the title, suffers from a form of mental retardation; he lives in the garden shed owned by the local priest, Father Francis McKeen. McKeen's brother, Terry, is a local landscape gardener and employs Jobe to help him with odd jobs. When Father McKeen finds insects around the church altar, he blames Jobe and beats him with a leather strap for forgetting his chores.
While Dr. Angelo records audio notes about needing a human subject, Jobe is mowing his lawn. It turns out that Peter, the young son of Angelo's neighbors, is friends with Jobe. Angelo invites them to play some virtual reality games and persuades Jobe to participate in his experiments, telling him it will make him smarter. Jobe agrees and begins a program of accelerated learning, using nootropic drugs, virtual reality input and cortex stimulation. Dr. Angelo makes it a special point to redesign all the intelligence-boosting treatments without the "aggression factors" used in the chimpanzee experiments.
Jobe soon becomes smarter, and Dr. Angelo starts taking Jobe to his lab at work to use the technology there. Jobe begins having sex with a young rich widow, Marnie, during his daytime job; he learns Latin in an hour-and-a-half at the lab at night. Jobe starts to have telepathic and hallucinatory experiences, but continues with the experiment at the lab, until an accident makes Angelo call a halt. The project director, employed by a mysterious agency known as The Shop, keeps a secret watch on the progress of the experiment, and soon swaps Angelo's new medications for the old Project 5 "aggression factors".
Jobe acquires telekinetic powers and takes Marnie to the lab to have sex with her in virtual reality; but something goes wrong in the simulation and Marnie cannot move and starts to panic. The experience is so traumatic that she is permanently brain damaged. Jobe's powers and abilities continue to grow, although the treatments seem to be affecting his mental stability, and soon he takes revenge on those who abused him when he was "dumb"; Father McKeen is engulfed in flames, a young man named Jake is tortured by a "lawnmower man" continually mowing his brain, and Jobe directs a real lawnmower to run down Peter's abusive father. Jobe makes the investigating police officers attribute it all to "bizarre accidents".
Jobe believes his final stage of evolution will be to become "pure energy" in the VSI computer mainframe. He plans to enter the VSI computer and from there reach into all the systems of the world, and he promises his "birth" will be signaled by every telephone on the planet ringing simultaneously. The Shop sends a team to capture Jobe, but they are ineffective against Jobe's abilities. Jobe returns to VSI, where he creates millions of virtual insects to attack the guards, and drives straight in. He confronts the director of the project and tortures him before using the lab equipment to enter the mainframe computer.
While inside the computer the network connections are disabled and Jobe is trapped in the deceased mainframe looking for an escape code. Angelo primes bombs to destroy the building and joins Jobe in virtual reality to have a minor argument with him. Jobe breaks Angelo's virtual prison and spins around and recovers himself before Jobe easily overpowers Angelo and proceeds to crucify him, then continues to search for a network connection. Peter runs into the building; Jobe still cares for him, and allows Angelo to go free in order to rescue Peter. Jobe finally escapes through a Maintenance Line as the building is destroyed in multiple explosions.
Back at home with Peter, Angelo and Peter's mother (who has implicitly become a romantic interest) are about to leave when their telephone rings, followed by the noise of a second, and then hundreds, all around the globe.