After the credits we open in a laboratory/control room for launching and monitoring satellites. Ellen Peters (Karen Kadler) announces that an unidentified object has been spotted. Her colleague, Pete Shelton (Charles B. Griffith) states that all aircraft should have been out of the area twenty minutes ago. The project manager, Dr. Paul Nelson (Peter Graves) notes that a commercial airliner is just off course. They are three minutes from launch. In Washington, D.C., Secretary Platt (Marshall Bradford) is meeting with Dr. Tom Anderson (Lee Van Cleef) when General Carpenter (David McMahon) enters the office. Anderson is there to beg Platt to call off the satellite project. When Anderson asks about the first satellite launched, he is told it exploded in orbit. Anderson tells them why, "It was a warning. I anticipated that three years before it was launched." He informs them that, "Alien intelligence watches us constantly." Much to his annoyance, Anderson is told the satellite has already been launched.
At the Anderson home, Tom and his wife Claire (Beverly Garland) are entertaining Dr. Paul Nelson and his wife, Joan (Sally Fraser). They are just finishing dinner. Nelson reports that the satellite has been in orbit for three months and all is well. Joan voices her annoyance that the men are always talking shop. Claire's concern is more directed at her husband and what she feels are his delusions. The girls work on the dishes while the boys examine Anderson's new radio. He tunes in a station and tells Nelson that the sound is the planet Venus. Within the hum and static he tells Nelson to listen to the voice. At that moment, the phone rings. It is for Nelson; the project is calling to tell him the satellite left its orbit and flew out into space. The Nelsons hastily excuse themselves and drive to the nearby installation. The military lead, General Pattick (Russ Bender) arrives first. Pvt. Manuel Ortiz (Jonathan Haze) and Sgt. Neil (Dick Miller) clear him through the gate.
The Andersons are in the living room. Claire is upset that her husband made a fool of himself in front of the Nelsons. The Nelsons arrive at the Installation and are cleared through. When Nelson enters the lab he's told by Pete Shelton that "its back". Pete, Ellen and Dr. Nelson exit the lab. Nelson tells them to get the reports to Washington before morning and that they'll bring the satellite down for a full examination. Nelson and his wife then drive home. It is early in the morning and Anderson gets out of bed and tunes in the radio set up in the living room. He's talking to something on board the satellite. Claire comes out to ask him back to bed. He tells her the creature from Venus is on board and will be coming to Earth to save us. Claire is exasperated; She is convinced her husband is crazy.
The next day the team brings the satellite back, but something goes wrong. Instead of the designated landing area, it crash lands ten miles south of the Anderson home, at a location near a cave and hot springs. When Anderson tells his wife, "he's alive, he survived the crash." She responds, "Tom, youre a sick man." She tells him she's going into town and hopes he's better when she returns. The Venusian creature exits the satellite. It looks like a giant squash or pickle with horns and claws on long arms. Almost immediately electricity all over the world fails. The Nelson car stops on the road. At the installation the power is off and the phones are dead. Ellen Peters notes how quiet it is. Floyd Mason (Paul Harbor), her colleague, reminds her that it is always quiet in the mountains. Floyd tells the general that the satellite is down; the signals were coming through before the power failure. The Nelsons note the time as 3:03 p.m. when all the clocks stopped. They decide to walk up to the Anderson home to call a garage. Anderson is reading off names to his Venusian master. Eight people are on the list for mind control devices. The creature expels bat like flying objects that will home in on their prey. In town people are beginning to panic. The crowd asks Haskell (Thomas E. Jackson) the newspaper editor what's happening in the rest of the world. Haskell tells them he doesn't know; the wire service broke down. A woman asks Sheriff Shallert (Taggart Casey) what she can do about her husband's iron lung machine. Anderson comes into town to collect his wife and is attacked on the street by a man. The sheriff breaks up the fight. Nelson and Joan are closer to the Anderson home; they stop to rest. They notice one of the bat-like creatures flying. Nelson throws a rock at it, but it misses. The Andersons drive home. He has the only working vehicle on earth. He tells her that electricity and combustion engines have been disabled. Even the water won't work. Claire is incredulous; she turns on the hose on their house and nearly hits her husband with the stream. He tells her, "That works because it belongs to me."
As the Sheriff is pushing cars off the road in town the bat-creature stings him leaving an electrical device imbedded in his neck. Like a bee, once it has stung its target it dies. The victim immediately disposes of the dead creature. The Nelsons arrive at the Anderson home and are greeted on the steps. They are invited in for a drink.
With all vehicles dead, General Pattick walks to headquarters. Along the way he encounters one of the bat-creatures. It stings him, and like the Sheriff he disposes of the dead creature. And like the Sheriff, he is now under the control of the Venusian.
Anderson tells Nelson the story so far, but Nelson doesn't believe him. Nelson does ask why Anderson isn't fighting it. Anderson says that his friend is here on earth to rescue mankind not to conquer. Nelson and his wife get into the Anderson car while Anderson calls the Venusian to tell him to trace the energy from the car to locate and possess Dr. Paul Nelson and his wife, Joan. The Andersons drive the Nelsons home.
At the installation, General Pattick tells the technicians that, "Were in the midst of a Communist Uprising, they've sabotaged every power source in the area." He orders the staff to remain at the installation. The city of Beachwood is evacuated. Nelson takes his bike to the installation, but no sooner does he leave but a bat creature stings his wife, Joan. As he is biking through town he notices the Sheriff order Haskell out of town. When the newspaper editor refuses he is shot dead by the Sheriff. Nelson demands an explanation. When the Sheriff tells Nelson he is going to place him under protective custody, Nelson slugs him. The Sheriff cold cocks him with his gun, but before he can kill him gets a message from the Venusian to stop. He tells Nelson, "you're to be one of us...Get up, you're free." Nelson gets back on his bike and leaves.
Anderson tells his wife about the control devices--the bat-like creatures. They plant a radiological device in the subject's neck. He tells her that once infected they are subjects of the benefactor and lose all emotions. Claire is horrified by such a condition.
Nelson arrives on his bicycle to find the installation closed. General Pattick is there to greet him. He tells Nelson that the staff was relocated to a nearby air base and that he and his wife should join them. He offers him a ride in the jeep and they put his bike in back. Nelson notices the jeep is running and suspects something. He invents a diversion, and hits the general on the back of the neck with his gun. He tosses the general out and commandeers the jeep. He drives to see Anderson and confronts him in his home. He calls him a murderer. Anderson tells him about the Venusian and the part he played. Anderson tries to convince Nelson about the benefit the alien proposes, but Nelson isn't buying any of it. Nelson leaves after calling Anderson a traitor. Claire tells her husband that he barely avoided being killed. Nelson had a gun. She chides him, "you just had an undeserved stay of execution." He contacts the alien, but it refuses to meet with him. No sooner has Nelson's lack of control been reported, but his commandeered jeep stops running. Fortunately he has his bike in back and peddles home. It is dusk when he arrives, but he doesn't notice the lights are on, both outside and inside the house. His wife greets him. She has a control device and throws it at him, then leaves the house for a walk. Nelson fights it off and manages to kill the thing. As soon as the control device is killed Anderson calls Nelson and invites him back over to his house to talk. Nelson tells him he has something to do first. His wife comes home and he pretends to be controlled. She tells him they will be controlled for the rest of their lives. At this, he shoots and kills Joan.
After his phone call, the alien tells Anderson to kill Nelson. Claire interrogates her husband about the Venusian. She tries her best to dissuade him from his grim task. He tells her the Venusian is established at Elephant Hot Spring cave because he needs a climate like Venus. He also tells her that Joan has been controlled. Two of the remaining devices have been used. The original targets were the mayor and his wife, but they were killed in the evacuation. Anderson won't tell his wife, but they were used on Pete and Floyd at the installation.
At the installation, Ellen wakes up and notices all the equipment functioning. She goes to make coffee and finds the two dead control devices. Floyd kills Ellen while Pete looks on, unemotional. Nelson arrives at the Anderson home in Joan's station wagon. Claire makes her final appeal to her husband, but to no avail. He leaves the house to meet Nelson outside. Furious, she contacts the alien on the radio and threatens it, "Do you hear that, I'm going to kill you." She grabs the rifle that Anderson placed on top of the radio and leaves the house through the garage. As Nelson and her husband walk up the stairs into the house she steals Joan's station wagon and departs armed and ready for a confrontation with the Venusian. She drives full speed to the cave.
Nelson informs Anderson that his wife is dead. "I killed her...not my wife, she wasn't my wife; She was a product of your work." He threatens Anderson, but wants information first. While Nelson interrogates Anderson, Claire arrives at the cave and enters looking for her quarry. As Anderson goes to retrieve his rifle he notices it is missing. Claire finds and confronts the Venusian, "you're ugly, horrible...go on try your intellect on me...you think you're going to make a slave of the world...I'll see you in hell first." She fires round after round with no effect. It kills her while her husband hears it on the radio at home. That changes things for Anderson. He tells Nelson that he'll take him to the installation, that Pete and Floyd are controlled. He will go to the cave and kill the creature.
Pvt. Ortiz, while out foraging for food, finds Joan's station wagon just outside the cave and hears Claire's screams. He enters the cave and fires at the creature, but manages to escape and return to his unit to report. Nelson enters the installation grounds and finds Ellen's body outside. He enters an office and shoots Pete, Floyd and General Pattick. He only manages to wound Pattick, but steals his working jeep. Anderson, in his own car, encounters the Sheriff blocking and guarding the road to the cave. He fires at Anderson, but Anderson circles around on foot and attacks him with a portable welding torch. The military detachment enters the cave. Gen. Pattick jumps into another jeep and heads for the cave. Nelson's jeep stops, he gets out and starts walking, but hears the general's jeep approaching and ambushes it. He shoots and kills the general and drives the jeep to the cave. The military unit opens fire on the Venusian, but it has little effect other than drawing it outside. Even a bazooka blast has no effect. Anderson pulls up in the commandeered police car and orders a cease fire. He lights up the torch and attacks the Venusian. It attacks and kills Anderson. They both die in a heap. Nelson pulls up in the jeep and gives his, "he learned almost too late that man is a feeling creature..." speech as we see the earth in space.