In 1928, a solitary mountaineer encounters a glowing sphere. He loses consciousness and when he wakes, the sphere has gone and there is a scar on his hand where a sample of his DNA has been taken.
In the present day, a rapidly moving object is detected beyond Jupiter's orbit and forecast to impact Manhattan. Moving at 30,000 kilometers per second, enough to destroy all life on Earth, the United States government hastily assembles a group of scientists, including Dr. Helen Benson and her friend Dr. Michael Granier, to develop a survival plan.
As it nears the planet, the object slows down just before impact. Revealed to be a large spherical spaceship, it lands gently in Central Park. The sphere is quickly surrounded by NYPD and heavily armed US military forces. An alien emerges and Helen moves forward to greet it; but amidst the confusion, the alien is shot. A gigantic robot appears and temporarily disables everything in the vicinity before the wounded alien voices the command "Klaatu barada nikto" to shut down the robot's defensive response.
The alien's exterior is found to be a bioengineered space suit, composed of placenta-like material covering a human-like being. The being quickly ages into Klaatu, who looks like the mountaineer from 1928. Klaatu informs Secretary of Defense Regina Jackson that he is a representative of a group of civilizations, sent to talk to the leaders of Earth about saving the planet. When Jackson instead sends him to be interrogated, Klaatu escapes and reconnects with Helen and her stepson, Jacob, telling them that he must finish his mission to "save the Earth".
The presence of the sphere, and other smaller ones that begin to appear all over the world, causes widespread panic. The military launches a drone attack on the Central Park sphere, but are thwarted by the robot. The military takes a weapons-free approach, cautiously enclosing the robot, soon nicknamed "GORT" (for Genetically Organized Robotic Technology), and transporting it to Mount Weather, an underground facility in Virginia.
Klaatu meets with another alien, Mr. Wu, who has lived on Earth for 70 years. Wu tells Klaatu that he has found the human race to be destructive, stubborn and unwilling to change, which matches Klaatu's experiences. Klaatu orders the smaller spheres to collect specimens of animal species, to preserve them for later reintroduction to the Earth. He clarifies for Helen that he means to save the Earth from destruction by humankind. When a policeman attempts to take them into custody, Klaatu kills him then promptly revives the officer, telling Helen and Jacob that he did this to simply disarm an obstacle to his mission.
Hoping to persuade Klaatu to change his mind about humanity, Helen takes him to the home of Professor Barnhardt, a Nobel Prize winner. They discuss how Klaatu's own race went through a drastic, collaborative evolution to prevent the demise of their own planet. Barnhardt pleads that Earth is at the same precipice, and humanity should be given a chance to understand that it too must change. While the adults are talking, Jacob calls the authorities to come and arrest Klaatu.
While the military is examining GORT, the robot transforms into a swarm of winged, insect-like, nano-machines that self-replicate as they consume everything in their path. The swarm soon devours the entire facility, emerging above ground to continue feeding.
The military capture Helen while Klaatu and Jacob escape on foot. As they travel, Klaatu learns more about humanity through Jacob. When Jacob contacts Helen and arranges to meet at his father's grave, the Secretary sends her to try and change Klaatu's mind. At the grave, Jacob is heartbroken that Klaatu cannot resurrect his long dead father. As Helen and Jacob have a tear-filled reunion, Klaatu's cumulative observations of humans convinces him to stop the swarm.
Dr. Granier drives them to the Central Park sphere, but the swarm has reached massive proportions. Klaatu trudges through the swarm to the sphere, touching it moments before his own body is consumed. The sphere deactivates the swarm, saving humanity, but at the expense of electrical activity on Earth, per Klaatu's warning that there will be a price to the modern human way of life.[6] The giant sphere leaves the Earth.
Who/what does Klaatu think is destroying the Earth?
Humankind