A map zooms into a small island in the Weddell Sea close to the Antarctic Circle. A huge volcanic eruption occurs and the narrator (Paul Frees
) explains for every possible action there is a reaction. The camera then tracks across the map to a point close to the North Pole. We see huge areas of ice breaking up and melting rapidly.
Revealed by this sudden melting is the body of a giant praying mantis. At a nearby radar station (Red Eagle One) part of the DEW line early warning system we see Col Joe Parkman (Craig Stevens
) arrive on one of the first flights to the new base.
Further north at an advance warning radar station (Weather four); two technicians pick up something odd on the radar. Before they can figure out the situation the whole installation is destroyed. Aircraft are dispatched to investigate, the pilots report heavy damage and the roofs been destroyed.
Hearing the reports Parkman flies up to inspect the damage. As reported the building is totally destroyed, all the electronic equipment is smashed. Parkman is puzzled he cant find any remains of the two men who were stationed there. Checking the perimeter he finds a series of strange marks scratched into the snow.
Back at Red Eagle, Lou the Radar Man (Phil Harvey
) picks up a strange intermittent contact by radar.
Taking no chances Parkman sends up fighters to intercept the contact, before they can get there the object disappears.
Later a C47 transport plane flying in the same area is attacked and destroyed. Search and rescue begins and ultimately the planes wreckage is found on a glacier. Parkman decides to investigate the wreckage first and discovers similar markings as at the weather station. No bodies are recovered from the crash site but a large horned appendage 5 feet long is discovered embedded in the fuselage.
The situation is relayed to CONAD in Colorado Springs. General Mark Ford (Donald Randolph
) is briefed of the ongoing mystery and the strange appendage found. Ford calls a conference of scientist to try and identify the object. Prof Anton Gunther (Florenz Ames
) is elected spokesperson. They know the item is organic, but the identity of the creature it belongs remains unknown. Gunther suggests contacting Prof Ned Jackson (William Hopper
) one of the leading paleontologists in the world.
At the institute where Jackson works Marge Blaine (Alix Talton
) visits him in his lab. Shes brought a layout of an article outlining the progression of life she intends to publish in the next issue of her magazine. During the discussion Ford rings Jackson and arranges a visit.
Jackson examines the item and suggests its not bone but cartilage. Jackson then extrapolates from the structure of the cartilage it came from an insect. Gunther recalls recovering fluid samples with the appendage and when tested confirms Jacksons theory.
Back at the institute, Marge shows Jackson a newspaper and begs him for information; she wants the chance to run a scoop in her small museum magazine
Gunther contacts Jackson to confirm the tests. Jackson explains that in prehistoric times insects grew to much larger sizes. He offers fossils of 2 foot dragonflies as evidence. Jackson ponders that if a creature was frozen quickly enough, there might be a possibility that if the creature thawed out, it may be able to reanimate. Jackson believes the insect theyre looking for is a Praying Mantis and the appendage they found is a hook section of its forelimbs
Meanwhile in Greenland, Eskimos villages see the beast, and try to evacuate to safety. The Mantis attacks the village eating a number of inhabitants.
In the US, the press is clamoring for information about the attack. Jackson refuses to say anything other than he has a theory, but needs to wait for further evidence before telling anything.
Jackson and Marge are organized to visit the location of the latest attack. He begins by visiting Parkman at Red Eagle One. They then fly to the crash site and Jackson sees the strange marks first hand which when measured are over 8 and a half feet long
Night falls at Red Eagle One, unseen by anyone the Mantis has landed attempts to attack the base. Marge sees the insect and gives the alarm. Base personal go to combat stations and drives off the threat using flame throwers and machine guns. Nine hours later constant air patrols have failed to locate any information to explain where the creature went after the attack.
Out to sea a fishing boat deep shrouded in fog is attacked and two of the deck hands are snatched off. The Mid Canada radar station contacts Parkman and Group Captains Hawkins reports an intermittent radar contact. Mapping the previous attacks confirms the creature appears to be heading south. Probably to the tropics where the vegetation is more akin to what it is used to
CONAD follows up with Parkman when a report comes through the creature has been sighted heading south from Newfoundland. The newspapers, now armed with real facts finally break the story. Public reaction is mixed, some panic, some think it is a hoax. The civilian ground observer corps is briefed and mobilized to watch for any signs of the creature.
An aircraft carrier patrolling along the Atlantic coast spots the creature and dispatches aircraft to intercept. The attack appears to be successful but overcast conditions stop the pilots from confirming the kill. Unseen by the aircraft the Mantis dips low into the clouds and flies on unaffected close to sea level.
Ned and Marge begin tracking any unusual occurrences across the country. They suspect with enough sightings, a pattern will emerge and help identify the progress of the giant insect.
Suddenly a report comes through of a train wreck in Maryland, an engine and five cars were overturned in whats described as a freak accident. Marge is not far from the site and heads over with Col Parkman to check the situation. Interviewing a local policeman all seems to be in order and the officer thinks it was simply caused by fog. After the investigation, Parkman takes Marge home, as the fog intensifies, he stops claiming it is too dangerous to drive. He really wants to show Marge how he feels about her. After he kisses her, she is non committal about the experience and insists they continue their journey.
A bus pulls up and a young lady gets off. Shes nervous about all the strange events but the driver assures her everything will be fine. The bus drives off and the woman screams as the Mantis ambushes the bus using the fog as cover. Rescue workers arrive to assist with the injured only to find the bus empty.
The emergency broadcast system reports the creature is heading for Washington and all defense personnel are to be on high alert. The insect flies flow over the city causing widespread panic before resting briefly on the Washington monument. After leaving Washington the bug begins to swing to the north east towards Baltimore. All along the flight path AA guns are ordered to fire on anything unexplained
The creature drops below radar coverage range and ground observer it to keep tracking. From all the reports coming in it is still heading North East and is closing on New York City. Again aircraft attempt to engage and destroy the creature
Taking part in the attack Parkman loses the creature among the clouds. Suddenly the creature appears right in front of him and rams the plane. Parkman survives the impact and bails out
The creature has been badly hurt by the accident lands in New York and crawls into the shelter of the Manhattan tunnel. The Tunnel is sealed and the army begins pumping smoke inside. Jackson believes the creature has been mortally wounded by the aircraft and it is only a matter of time before it dies. Gen Ford is concerned it might try to break out and agrees to allow Parkman to lead an assault on the creature.
Pushing into the tunnel they can hear the creature moving around, attacking some of the cars previously abandoned. Finally the creature sees Parkmans team and advances, they open fire with guns and special RP mines. It is clear they are slowing the creature down, but fail to kill it outright. A final assault proves successful and the creature dies.
While Parkman Jackson and Marge examine the creature, an auto reflex action from one of its arms nearly kills Marge. Parkman gets her to safety, this leads to a romantic moment between the two as the final credits role.