The main character is Herby Strange, an 11-year-old budding scientist. He and his family live in an old house, where Herby has a laboratory. Because of his great concern for the woes caused by world overpopulation and disappearing animal habitats, he is working on a formula to shrink people and animals so that there will be plenty of room for all.
The formula accidentally falls into Herby’s bath. He shrinks and goes down the drain. From the plumbing, he climbs a vent pipe to the roof. After a treacherous climb down vines and trellis, he reaches the ground and thrashes his way through vegetation toward voices on the front porch. By the time he reaches the porch, the two policemen have left, his aunt has closed the door, and his cries go unheeded. He has, however, heard the discussion about his supposed kidnapping.
Herby spends the night in a pickle jar, wrapped in newspaper, and unintentionally gets carried away by the recycling pick-up. He escapes being crushed at the recycling plant, and then makes his way to a park, where an old man who thinks Herby is a leprechaun captures him. He escapes the old man, only to be kidnapped by a drifter who wants him for ransom. The drifter hitches a ride on a “semi”, steals the truck, and then wrecks it on a rain-slick highway. Herby escapes and hides from the drifter in a culvert. Along with the highway patrol, Herby’s parents and his dog, Bozo, arrive at the accident scene, but Herby is carried away after the drifter tosses him into water going into a creek.
His own ingenuity, and the knowledge and training learned in Boy Scouts, save Herby from the many perils he faces on the long trek home through the mountain wilderness.
In order to cross a large lake that lies between Herby and home, he spends several days in a deserted campground, building a sailboat out of trash. He is caught in a storm on the lake and blown onto a small island where he spends the night. The perilous crossing ends the next day with his being swept over a spillway. An injury to his ankle, and a confrontation with a bobcat, dash Herby’s hopes of ever seeing home again. However, his dog has been trailing him, finds him, and provides Herby transport.
The starving dog finds food at a circus, and frightened by clowns, runs off into the night, with Herby on his back. The two happen upon the camp of Herby’s scout patrol, which has been searching for them. Herby learns what has transpired in his absence. He looks forward to being reunited with his family, but is concerned how they will react to his shrinkage.
There is much ado at Herby’s return. In hopes of finding an antidote to Herby’s shrinkage, the family doctor makes arrangements for Herby to be transported to a medical research facility in California. Before leaving, Herby makes an appearance on national television, which ends in hilarious mayhem.