Three years after test pilot Bruce Barton (Duncan Parkin) disappears while flying over the Tarahuamare Mountains in Mexico, his fiancée Susan Winter (Gloria Talbott) throws together an expedition to search for him, teaming up with wealthy stock speculator Marty Melville (Lon Chaney Jr.), biologist Russ Bradford (James Craig), and pilot Lee Brand (Tom Drake).
Disobeying orders from the Mexican government that the Tarahuamare Mountains are off limits, the gang piles into Lee's plane and flies off. Halfway to their destination, rough winds force the plane to make an emergency landing in a narrow valley.
It becomes evident that Bruce isn't the sole object of Marty's search. He's brought a radiation-detecting device with him (which he refers to as a "scintilator" and which resembles a hair dryer the prop department tricked out) in the hopes of locating uranium deposits so he can file a claim with the Mexican government as part of some kind of swindle/get-rich-quick scheme.
Russ sees a giant lizard, but nobody believes him. Then, searching for Bruce, he and Susan stumble across a giant mouse, which is then caught and eaten by an equally giant hawk. Russ speculates that radiation from the uranium deposits Marty is so obsessed with have caused the local wildlife to grow to gigantic proportions.
Monsters or no monsters, Susan is determined to find Bruce, and the gang presses on. But the giant animals aren’t the only dangers of the valley. The biggest mutant of all is an enormous man some fifty feet tall, with one side of his face partially melted, obliterating his eye, while his remaining eye is bulbous and glaring. Seemingly unintelligent, the "cyclops" bellows, roars and grunts, and becomes hellbent on capturing the interloping humans… especially Susan, who begins suspecting a connection between the cyclops and Bruce.
Tropes used in this film:
- All Love Is Unrequited: Russ loves Susan, but knows that she won't return his affections because her heart still belongs to Bruce.
- Antagonist Title: The title refers to the one-eyed giant that menaces the film's main characters.
- Artistic License – Biology: Russ explains that the reason everything in the valley is giant is because because radiation coming off uranium is overstimulating their pituitary glands, causing their cells to never stop growing. He even says if they remain in the valley too long, they'll eventually begin turning into giants, too.
- Artistic License – Nuclear Physics: With that much radiation pouring off of the ground, all the animals (and Bruce) should all be dead, not mutated.
- Attack of the 50-Foot Whatever: In addition to the cyclops, there's also giant birds, rodents, lizards and various insects and spiders. There's even a giant python.
- Big Creepy-Crawlies/Giant Spider: At one point, the group is menaced by a giant tarantula.
- Bronson Canyon and Caves: The cyclops' lair is none other than Bronson Canyon.
- Chekhov's Gun: The big stick Lee carries around. Russ fashions it into a makeshift spear and uses it to blind Cyclops!Bruce.
- Cool Shades: Lee wears some pretty cool aviators.
- Cool Plane: A Stinson 108-3 Voyager.
- Corrupt Corporate Executive: Marty wishes to make a fortune from the uranium and has zero regards for Susan's feelings for Bruce and desire to have closure.
- Cyclops: The titular monster is a giant with one eye, though unlike a conventional cyclops with the single eye in the middle of the forehead, he instead has one eye because the other is obscured by a wall of flesh as a result of the mutation.
- Distinguished Gentleman's Pipe: Russ can be seen puffing away on a pipe in some scenes, to remind us he's the intellectual type.
- Downer Ending: Russ blinds Cyclops!Bruce in self-defense, and then he, Susan and Lee escape in the plane, while Bruce apparently dies from the eye injury.
- Dull Surprise: Russ' reaction to the giant animals.
- Eagle Land: Unavoidable, given that it's about a bunch of Americans traipsing around Mexico and ignoring their no-trespassing laws. While most of the gang are Mixed Flavor, Marty is definitely America, the Boorish, a thoroughly unpleasant, rich jerk who solves problems with his fists.
- Eye Scream: Russ throws a fiery spear into the cyclops' eye during the climax.
- Dirty Coward: Marty tries to run away the moment the cyclops appears.
- Guns Are Worthless: Averted. The giant animals can be felled by ordinary rifle fire.
- It's played straight with the cyclops himself, though. He's Immune to Bullets and Marty unloads several rifle rounds into him and it just makes him mad.
- "I Know You're in There Somewhere" Fight: Attempted by Susan when she realizes the cyclops is Bruce.
- Impending Doom P.O.V.: At one point, we see Susan from the cyclops' perspective.
- It's All About Me: Marty seems to think the expedition is entirely about his quest for uranium: as soon as they arrive and he confirms the mineral's presence, he's all for flying back and forgetting about Bruce.
- Jerkass Has a Point: Marty's insistence that they give up the search and leave the valley is at first dismissed as pure cowardice on his part, but after discovering that it's the uranium that makes living things grow gigantic, Russ decides, yeah, Marty kinda has a point, and agrees with him.
- Madness Mantra: Susan does this after her first encounter with the cyclops. "The eye! The eye!"
- My Plane Hates Me: Whilst attempting to escape at the end, the plane's engine conks out.
- Nightmare Face: The cyclops' face◊ is quite horrific, similar to Glenn Manning in the later War of the Colossal Beast, also directed by Bert I. Gordon (it's even the same actor, Duncan "Dean" Parkin, as both monsters).
- Nuclear Mutant: Uranium-induced giants.
- Off-the-Shelf FX: Much has been made by critics about the "scintilator" Marty uses. It may or may not be some type of 50s-era radiation detector, but it looks more like a tricked-out futuristic hairdryer.
- Obstructive Bureaucrat: The governor at the beginning who refuses to allow the expedition to search for Bruce. In a subversion, he has his reasons. Specifically, he doesn't trust Marty Melville any further than he can throw him.
- Rodents of Unusual Size: One gets nommed by a giant hawk.
- Screaming Woman: Susan screams a lot at the sight of the giant animals.
- Slurpasaur: A number of the gigantic critters in the valley are these.
- Spiders Are Scary: A spider is one of the giant creatures the main characters encounter.
- Two-Faced: The titular cyclops has one side of his face deformed, with a lack of lips exposing the teeth on that side of his face and a blank wall of flesh covering one of his eyes.
- Was Once a Man: The cyclops is actually a mutated Bruce.
- What Happened to the Mouse?: The tarantula just up and disappears after briefly menacing the heroes. It's there and gone between shots in the same scene.