On June 12, 1983, suborbital passenger flight 612 (or Spindrift), on its way from Los Angeles to London, encounters a bizarre storm and is forced to make a crash landing.
Steve Burton (Gary Conway), co-pilot Dan Erickson (Don Marshall), stewardess Betty Hamilton (Heather Young), and passengers: engineer Mark Wilson (Don Matheson), pampered city girl and jet-setter Valerie Scott (Deanna Lund), bank robber (disguised as a naval commander) Alexander B. Fitzhugh (Kurt Kasznar), orphan boy Barry Lockridge (Stefan Arngrim) and his dog Chipper – find themselves in a mysterious land dominated by giants.
When the others search for him, they discover an abandoned town built to human scale, but soon learn it is an elaborate model created by an old man named Akman.
Steve and Fitzhugh attempt to steal a lens from a camera they see set down by a photographer, but then witness the man murder his photo subject – a young woman who resisted his advances.
A drunken hobo enters the park and passes out on a bench, giving the killer the opportunity to plant evidence and make it look like the bum committed the crime.
Barry and Fitzhugh are captured by a gypsy boy named Pepi, who hands them to his uncle Carlos, a carnival owner, who plans to sell them to a circus.
The Earthlings find a miniaturized tape recorder that leads to a crazed human astronaut named Major Kagen, who thinks they are spies for the giants.
While hiding in a drain pipe, Steve and Fitzhugh find a room full of electronic equipment on a human scale, which they learn is from a lost American space flight.
Taking advantage of the fact that the giants have poor night vision, the humans sneak into a laboratory after disabling the lights, but a scientist spots them with a pair of experimental infrared goggles.
Inspector Kobick, an officer with the police's Special Investigations Department (SID), develops a way to track the humans using a scanner that detects a metal unique to Earth.
He is saved for the moment by an organ grinder, who is hunting the little people for the reward now offered for them, but the demented man captures the orphans and threatens to harm them if they will not reveal where more Earthlings can be found.
Steve comes up with a plan to slip an animal tranquilizer to the guard, but the attempt is thwarted when Kobick is tipped off by Grotius, a dishonest handyman, who is trying to extort a reward for the capture of the Earthlings.
Overhearing the scientist, Dr Franzen, say he is working to peacefully explore the solar system, Mark proposes a deal: to perfect the device in exchange for a ride back to Earth.
Seeking shelter from a storm, the little people enter a haunted house, but soon realize the green phantoms they see are illusions, created by a magician named Inidu, who was only trying to drive off a pair of trespassing boys.
His parents, facing financial problems, decide to turn the Earthlings in for the reward, and with the boy's keen eye, they follow the little people's footprints back to the Spindrift and capture the ship, as well.
To find the criminals, Kobick puts the Earthlings into the city's sewer system, to follow a trail of luminous ink used in printing the fake money.
An insane grand master named Kronig captures the little people, but proposes a deal with them – play him at chess and if they win, they go free; lose and he turns them over to the SID.
Unbeknown to Biff, Mark has rigged the phone and Steve intercepts the call to SID and buys the others some time, but Dan tries a different approach – make Bowers a star overnight by teaching him Earth's jazz, in exchange for letting his companions go.
Chipper is injured when the bark from a giant dog sends him tumbling into a wall, and Barry risks capture by taking him to a veterinarian at a pet store.
They show up at the Earthlings' camp just as a giant stumbles upon the Spindrift, but the futuristic humans kill the intruder and disintegrate the body with an amulet weapon before anyone realizes what happened.
Steve, Mark, Valerie, and Barry become caught in a toy balloon and end up pulled by a strange force over a thousand miles across a violent ocean to a land ruled by a ruthless dictator named Titus.
Soon, however, the pod is caught in an energy storm that takes Steve and Fitzhugh back to a small New England town on Earth, but stuck 75 years in the past.
As the weird effect begins to grow out of control, it creates strange warps in space and time, where the giants become invisible to the humans and Steve becomes trapped in a nightmarish dimension.
With the others' help, Mark comes up with a plan to contaminate the soil at the dig site with an explosive chemical that would prevent the surface giants from blasting and discovering the hidden city.
Suddenly, they are teleported away to the home of Professor Kirmus, who has developed a matter transport device capable of sending anything anywhere, including the little people back to Earth.
Steve tries to reason with the boy, but is unsuccessful; after Alek "bombs" the Spindrift camp with stones from the plane, a fed-up Mark attempts to stop the child even if it means having to harm him.
Steve and Dan encounter two time-travelling people their size, Throg and Berna, who escape an SID agent by using an amazing device called the "Space-Time Manipulator" (STM).
Throg and Berna soon arrive and angrily explain that it is forbidden to change history, but Steve and Dan do not want to go back to the giant world so easily.
Steve, Dan, Valerie, and Fitzhugh are caught by a mad scientist named Melzac, who puts them in a model plane and flies it to the "Graveyard of Fools" – an uncharted land on the giants' world from where no one has ever returned.
Meanwhile, Mark is caught by Melzac, who reveals his brother and his plans to fix an alien device, the "servo actuator", which is causing ripples in space and time and requires the use of the little ones to get inside it and conduct repairs.