While driving through the desert with his wife Carol Marvin to a military base to send the eleventh rocket into Earth orbit to assist the exploration of outer space in Operation Sky Hook, Dr. Russell A. Marvin and Carol see a flying saucer and accidentally records a message on their tape recorder. Once in the base, Dr. Russell is informed by his father-in-law and general that the ten first satellites mysteriously fell back to Earth. When Dr. Russell decodes the message, he encounters the aliens, who ask him to schedule a meeting with the leaders of Earth in Washington in 56 days in order to invade Earth without panicking the population. Dr. Russell develops an anti-magnetic weapon that becomes the last hope of the human race against the hostile aliens.
We see a montage of UFO sightings including Air force pilots, farmers and civilians. After collecting and examining these reports the military decides they are a threat and orders a shoot on sight orderThenDr. Russell Marvin (Hugh Marlow) and his recently married wife Dr. Carol Marvin (Joan Taylor) are driving through the desert to a rocket testing ground – Project Skyhook, when they are buzzed by a UFO. Although they don't realize it at the time, a recorder used to dictate research notes has picked up and recorded the UFO encounter.General John Hanley (Morris Ankrun) arrives at the rocket range and begins trying to postpone the launch. However Marvin and other scientists advise it is too late to abort the launch. During this conversation we discover that Carol Marvin is his daughter, and although pleased for the couple was unaware of their recent marriage.Rocket 11 launches without incidentThat evening over dinner the General discusses with the Marvins the failure of the previous 10 rockets launched in the program. Dr. Marvin surmises that UFOs might have shot the rockets down; he then reveals the recording he made during his encounter with the UFO. As dinner is finishing the three of them see glowing lights in the sky, Carol explains they are fog lights, and no one knows what they are. A few seconds later the wreckage of rocket 11 enters the atmosphere and burns up.Against everyone's better judgment, it is decided to launch rocket 12. During preparations for the launch a UFO is seen buzzing the rocket range before landing in the middle of the facility. Three suited occupants emerge and are fired upon by soldiers. The aliens retaliate, as one of their group is killed. Returning to their ship with their fallen comrade the aliens take off and begin to systematically destroy the rocket test area, kidnapping General Hanley in the processTrapped below ground by fires started during the attack, the Marvins begin to consider rescue unlikely. With their air running out, they decide to make a recording of the events that led up to the alien attack. The tape recorder they are using starts to lose power slowing the playback down. Rewinding the tape the Marvins realize the noise they heard was greatly sped up message from the aliens asking them to meet at the rocket installation. Dr. Marvin understands that a terrible mistake has occurred and fears what might come of it.To their surprise the Marvins are rescued and race to Washington to reveal what they have learned. The authorities expressly forbid Dr. Marvin from making contact with the aliens. Ignoring this, Marvin does contact them and gets new instructions for a meeting place: a beach on the California coast.Carol and Major Huglin (Donald Curtis) discover his plan and try to stop him, racing to the rendezvous point, where the aliens have already landed. Marvin ignores them and approaches the ship. A voice explains it is safe to enter, which they do.Once inside the ship a disembodied voice explains the aliens' mission. They come from a dying star and need to find a new place to live. They demand that Earth surrenders and let them control of the planet. The aliens then admit they have Carol's father onboard and have sucked all memories out of his brain, reducing him to a zombie-like state. Finally, they give Earth an ultimatum of two lunar months to surrender or they will attack with their fleet. Once this is understood, the humans are released, except General Hanley.Back in Washington, the group is debriefed over events on the ship. Further debate breaks out over exactly how to handle the situation. Dr. Marvin explains given his interactions with the aliens he might have an idea for a weapon that could stop the attack. The military embraces the idea of a weapon that uses sound to disorientate UFOs causing them to crash. With only sixty days to the attack the military begins talking to other nations to help speed up and build enough weapons to fend off the attack.Weeks pass and the development is better than expected. During a final test of the weapon a spy drone sent by the aliens is spotted. During a final test of the weapon, an alien spy drone is spotted observing the area, the scientists destroy the drone, but fear the aliens now have valuable information regarding the weapon and its location.The scientists evacuate but during the final stages of the process an alien ship arrives. Seeing an opportunity the weapon is deployed against the ship. The ray works and the aliens are driven off, but not before one of them is killed and left behind.Opening the alien's suit reveals a highly atrophied corpse, it is thought that the aliens are quiet fragile and need the suits to survive. In a matter of minutes the corpse rots and disappears to nothing. Before any more can be done the UFO returns and destroys the installation where the weapon had been housed. As the UFO leaves after the successful attack, two bodies are seen falling from the craft. One of them is Carol's dead father, General HanleyThe next day further examination of the suit reveals a communication device, with which the military is able to interpret the attack plan of the aliens. It also appears to be able to amplify both sight and hearing. Examining how the attack succeeded the day before, a disadvantage is discovered in that the ray only has a range of 1500 yards.The aliens interrupt the discussion with a worldwide broadcast threatening to manipulate the sun in such a way as to cause severe meteorological convulsions in eight days . The aliens insist that all the world leaders should meet in Washington to discuss surrender termsThe military believes rather than discussing anything the aliens intend to destroy Washington. As predicted, the solar eruption takes place. It is further realized that there are only nine days to prepare, and those preparations are going to suffer due to worsening weather conditions.We now see a montage of the increasing disruption by the weather. This causes delays evacuating civilians, and there is still 60% of the population still in the city when sirens begin to sound indicating the attack has begun.The aliens brush past a combined aircraft and artillery attack and three crafts begin to assemble over Washington. Dr. Marvin fires the sound weapon and downs one of the alien craft. Artillery batteries open fire again to distract the remaining ships as Dr. Marvin repositions his weapon to attempt a further attack.The aliens respond by opening fire on various buildings around the White House and Capitol building. Dr Marvin races with their team to re-deploy and defend the White House. A second ray unit is destroyed and the aliens land. Local soldiers attempt to slow down the invaders as the second machine deployed.The two remaining ships are brought down in quick order. On the other side of Washington more ray units deploy and yet another ship is brought down crashing into the Washington monument. A running battle breaks out around the Supreme Court trapping the Marvins amid the wreckage.An emergency broadcast announces that more saucers are landing in front of the Capitol building. A battle intensifies as more ships appear and the aliens leave their craft in one final assault. This is beaten back with a combination of ray machines and infantry. Suddenly everything goes quiet and slowly the realization comes that the battle is over and the aliens have been repelled.Some time has passed and we now find the Marvins relaxing on a beach. Mrs. Marvin reads a newspaper article that her husband has been placed in charge of rebuilding Project Skyhook and that the United Nations is going to award him a gold medal.The two go for a swim in the ocean as the final credits roll.