Chased by Kobick and a sergeant using dobermin pinschers, Steve manages to get Dan and Mark into a vent at an extra large building. He runs into a second vent, intending to lure the dogs away from the vent Dan and Mark are in. But Kobick to check the other end of the vent inside the building. Dan and Mark make their way back toward the outside, thinking both giants might be gone, only to have both dogs rear their snouts close to the vent, barking viciously at them.
Dan and Mark move to another opening and go inside the building area. They hide in a prototype machine of the scientist Franzen and his doting wife Altha. During a test of the machine, the vents are shut tight and the machine is also. Dan and Mark are trapped inside with radiation bursts about to start which would kill them. Kobick storms in without regard for the test or his dogs, which could have been hurt by the machine. He apologizes telling the two that the chase after the little people doesn't leave much time for the humanities." Steve gets out of his vent and sneaks into the other room where the test is being conducted and just in time, saves the two men. After he and Dan slide down the wire to the device, Mark stays in the test room and hears the two giants talk to fiancier Logar. The device is for a projected spaceship. They want to contact other worlds including Earth, Venus, and Mercury. The prototype is a guidance system. Franzen seems to only want good will between the people of other planets and their own people. Mark, back at camp, shows Steve and Dan his plans to fix the giants' system. Their conversation ends up in a strong debate involving the entire group with Steve and Mark getting angry at each other and with Mark asking for others to help him. Fitz and Val go, wanting to take a chance. Steve tells Mark not to have any giants trailing back to camp. Later, Steve follows the rebellious trio and gives them radios, apologizing for both of them getting so hot, but still standing by his descision. Val and Fitz explain to Steve why they are helping Mark. Val tells Steve she is sick and tired of running and hiding and living like an insect in this giant grotesque world and tells him to stay here. She wants to go back home. Steve leaves and Mark has the pair wait outside while he makes contact with the giants, who agree to the plan: promise them a free ride back to Earth and Mark will fix it. When Logar tells Altha that the defense department supercedes the SID, she puts it to the test and calls Inspector Kobick on the phone.
Logar stops Kobick from caging the little people, demanding they let the Supreme Council decide; then behind closed doors, makes a deal with Kobick: he can take them after they have used them to fix their device. Betty talks to Steve and gives Dan a look as if he should also. The two talk, not sure about their stand on Mark's plan. They wonder if Mark could be right but feel it is too dangerous to do what he is doing. Altha traps Mark, Fitz, and Val in the prototype as gas pours out of it. Val claims they are going to be roasted alive.
Fitz's hands are burned when he couldn't hit a pipe that Mark was holding and Mark hit a pipe that Fitz decided to hold instead. Franzen, in the other room, gets an alarm from Mark's move and gets them out. Val and Fitz leave, his hands bandaged. They called the spaceship but Steve tells them to meet at a tree. Fitz tries to get Steve to agree to help Mark but Steve continues to resent it and doesn't trust the giants. Just then, two giants, one a new Sergeant and the other, an SID man, tower above them, with two giant dobermin dogs.
Steve and Dan lead the giants and the dogs away from the others, who slip under a tree root and get away. Steve and Dan aren't so lucky: they run to a gopher hole where the giants make the dogs stick their noses in, reach in themselves with their hands, fire blanks from their guns into the small cave within, and finally smoke them out with a giant flare. They are dropped onto Franzen's table in no time, Steve punching Mark, thinking Mark made the giants do this. At camp, Betty tries to cheer up Fitzhugh but later worries when she and Val don't hear from the men. Steve slides down the wire but Mark and Franzen implore Dan to stay and help. He does. Franzen also guesses the truth about his wife, who didn't want the little people to get the credit for all the work he put into the project. Dan and Mark fix the device--and Logar puts them in Kobick's cage, ready to phone the SID. Steve climbs up to the table as Mark tells Dan, "Giants are no different than we are. Some of them can be trusted and some of them can't." Steve frees them but Logar runs into the room.
Steve hits the destruct button which Mark told him what it was. Franzen fights with Logar and opens the vent, allowing the three Earthlings to run out. Mark tries to warn him about the self destruct. Logar is killed in the fight by accidentally falling into the live controls. As Franzen stoops to check him, the building seems to blow up with the good giant still in it. Steve tells Mark they will get back to Earth--but it won't be in one of the giants'