In the South Seas, a volcano explodes, eventually causing North Pole icebergs to shift. Below the melting polar ice caps, a 200-foot-long praying mantis, trapped in the ice for millions of years, begins to stir. Soon after, the military personnel at Red Eagle One, a military station in northern Canada that monitors information gathered from the Distant Early Warning Line, realize that the men at one of their outposts are not responding to calls. Commanding officer Col. Joe Parkman (Craig Stevens) flies there to investigate, and finds the post destroyed, its men gone, and giant slashes left in the snow outside.
When a radar blip is sighted, Joe sends his pilots out to investigate, but their intended target disappears. Soon an Air Force plane is attacked by the deadly mantis. He searches the wreckage, and this time, in addition to the huge slashes, finds a five-foot-long pointed object in the snow. He takes it to General Mark Ford (Donald Randolph) at the Continental Air Defense (CONAD) in Colorado Springs, Colorado.
Ford gathers top scientists, including Professor Anton Gunther (Florenz Ames ), to examine the object, but when they cannot identify it, Gunther recommends calling in Dr. Nedrick Jackson (William Hopper), a paleontologist at the Museum of Natural History.
When Ned gets the call from Ford, he is helping museum magazine editor Marge Blaine (Alix Talton) plan her next issue, and dodges her questions as she begs him for a big scoop. Later, after examining the object, Ned recognizes it as a torn-off spur from an insect's leg, and soon guesses, from evidence that the creature ate human flesh, that it must be a gigantic praying mantis. Meanwhile, in the Arctic, the people of an Inuit village spot the mantis in the sky, and although they hurry to their boats to escape, it swoops down and kills several men.
Ned is sent to Red Eagle One to investigate further, and upon leaving, discovers that Marge has managed to get permission to accompany him as his photographer. They reach the base, where all the men, including Joe, are smitten by Marge.
That night, Marge and Joe join Ned in his office and discuss the creature, not realizing that it is drawing close to the office window. Marge suddenly catches sight of it and screams, and the bug attacks the building. Although the full unit opens fire on the mantis with automatic rifles and a flame-thrower, it is unscathed and moves away only after aircraft encircle it.
Hours later, the base remains on red alert, but they finally hear that the bug has attacked a boat off the Canadian coast, which means, Ned calculates, that it is flying at a speed of 200 miles an hour. Ford calls a press conference to announce the bug's existence, and asks the Ground Observer Corps to track its whereabouts.
Over the next few days, Ned, Marge and Joe track the bug's progress with the help of military and civilian observers. Late one night, Joe drives Marge home, stopping briefly to ask for, and receive, a kiss. They are distracted by a report of a nearby train wreck, and although they assume it to be an ordinary accident, soon after, a woman leaving a bus sees the mantis, and all emergency personnel are put on alert. The mantis is then sighted in Washington, D.C., atop the Washington Monument.
Joe is one of the pilots who attempt to drive the mantis toward the sea, but a dense fog throws him off course, and he flies directly into it. As the wounded mantis drops to the ground and crawls into the Manhattan Tunnel, Joe safely parachutes to the ground.
Ford leads a team that seals off the tunnel, filling it with smoke to provide cover for Joe and his special unit of men, who enter the tunnel armed with rifles and three chemical bombs. They creep past wrecked cars until suddenly the bug appears in the fog only a few yards ahead of them. They shoot at it, but it lumbers on, forcing them backward. The mantis seems immune to the ammunition and the first chemical bombs until, only feet from the tunnel entrance, Joe throws a bomb in its face, and it collapses, dead.
Later, Ford, Ned, Joe and Marge enter the tunnel to examine the bug. Marge photographs its face while the men walk around its side, but Joe suddenly sees the mantis' arm move, and runs to protect Marge. Although Ned explains that the bug's movement was merely an autonomic reflex, Joe takes the opportunity to pull Marge into an embrace.
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