In 1928, on an expedition in the snowy Karakoram mountains of India, a mountaineer (Keanu Reeves) encounters a glowing sphere of ice.
After breaking away a chunk of ice with his pick axe, he is blinded by a fierce & ferocious light, then finds himself waking after a sudden loss of consciousness, with the sphere now gone and a scar on his hand.
Fast forward to the present day. A famed astrobiologist at Princeton University, Professor Helen Benson (Jennifer Connelly) goes home to make dinner for her step-son Jacob (Jaden Smith) when she receives an odd phone call that "they" are on their way to pick her up.
When asked who "they" are, the United States government pulls in front of her house and hastily tells her she is under protective services and must leave with them immediately. A neighbor volunteers to watch over Jacob and the security team speeds off with Helen.
During the ride, Helen demands a reason why she is being escorted, the agent explains it's a matter of national security.
Helen: National security? Well that just means whatever you want it to mean.
They take her to a helicopter where a number of scientists and specialists have also been gathered. They speculate on the purpose of this gathering, then the helicopter takes off. They arrive at Ft. Linwood Military Academy in New Jersey where soldiers are collecting cellphones and cameras. Helen hides her cellphone.
She is assembled with others in a conference room and is shown that a large unknown object with a speed of approximately one-tenth the speed of light is due to impact Manhattan in approximately 78 minutes. After the meeting, Helen hides in a bathroom stall and uses her cellphone to call Jacob.
A Guardswoman catches Helen with the cellphone after they had been forbidden.
Guardswoman: Is that a cell phone?
Guardswoman [chokingly]: Can I borrow it?
She is put on another helicopter along with a group of other scientists to formulate a survival plan. Just as they brace themselves for impact, all is quiet. Apparently it has slowed down and is not going to crash. Dr. Benson turns around to look out the window of the helicopter just in time to see a large bright sphere approach the ground.
It is a spaceship and it lands very gently in Central Park. A being named Klaatu (Keanu Reeves) emerges from the sphere, around which the military has established a perimeter.
Just as Dr. Benson is about to shake hands with the being, it is shot. A gigantic robot emerges, emitting a sound that paralyzes humans and disrupts all electrical systems in New York City.
The robot suspends its hand just above Klaatu and then deactivates, closing its optical beam. Klaatu is gathered up by the miltary and rushed to the hospital where the doctors revive the dying being. A surgeon is brought in to remove the bullet. While attempting to remove the bullet, he notices that the beings outer layer is a jelly substance much like whale blubber and it begins to fall off on its own.
The theory of this was is that it is the being's "placenta" much like a human baby and that it was "born" here on earth. In 1928 when it landed the first time, the patch of skin removed from the mountaineer's hand was a DNA sample so the being could replicate itself into human form. The being is immediately put into an incubator while it "grows".
Time passes and we see a full grown "human" inside the incubator. Once fully grown and awake he begins to thrash around in the incubator and the first person there to speak to him is Dr. Benson. While recovering he is interviewed by Regina Jackson (Kathy Bates), the United States Secretary of Defense, Dr. Michael Granier (Jon Hamm), and Dr. Benson.
Regina: My name is Regina Jackson. I'm the Secretary of Defense of the United States of America.
Klaatu: This body will take some getting used to.
Regina: I've been sent here to determine who or what you represent and what your intentions are.
Klaatu: It feels unreal to me. Alien. It will take time to adapt.
Helen: What were you before you were human?
Helen: Different how?
Klaatu: It would only frighten you.
He introduces himself as Klaatu and informs them that he represent a group of alien races.
Regina: What is your purpose in coming here?
Klaatu: There is a gathering of world leaders not far from here; I will explain my purpose to them.
Regina: I'm afraid that's not possible. Perhaps you should explain yourself to me instead.
Klaatu: Do you speak for the entire human race?
Regina: I speak for the President of the United States. Now, please, tell me, why have you come to our planet.
Klaatu: Your planet.
Regina: Yes, this is our planet.
Klaatu: No, it is not.
Jackson has him sedated and plans to move him to a more secure location with the possibility of interrogating him. The scientists feel that Klaatu should be allowed to address the U.N.
Regina: History has lessons to teach us about first encounters between civilizations. As a rule, the less advanced civilization is either exterminated or enslaved. I'm thinking of Pizarro and the Incans, Columbus and the Native Americans, and the list goes on. Unfortunately in this case, the less advanced civilization is us.
Granier: This is the representative of an extra-terrestrial civilization. This is the most important discovery in the history of Mankind!
Regina: It may well be the last discovery in the history of Mankind.
The scientists protest Regina's plan to drug him and interrogate him, and refuse to cooperate. However, Dr. Benson volunteers to do the honors of drugging him. Helen enters a drug supply room and selects a harmless saline solution. Regina tries talking with Klaatu again.
Regina: I want to apologize to you; it seems we got off to a bad start.
Klaatu: It seems we did.
When she informs him that they are going to transfer him to another facility, he refuses and insists he is leaving. Helen steps foward to give him the shot and injects him with the saline solution.
Helen [whispering to Klaatu]: Run.
Klaattu is taken into a room for a polygraph test.
Polygraph Operator: I'm going to ask you a series of control questions. Are you currently in a seated position?
Polygraph Operator: Are you human?
Klaatu: My body is.
Polygraph Operator: Do you feel pain?
Klaatu: My body does.
Polygraph Operator: Are you aware of an impending attack on the planet earth?
Klaatu: You should let me go.
Polygraph Operator: I'll repeat the question.
The Operator touches the machine and gets a shock.
Klaatu: What gate did you use to enter this base?
Polygraph Operator: East loading door.
Klaatu: How do I get there?
Polygraph Operator: 100 yards left from the back of this building.
Klaatu: Is there a security code?
Polygraph Operator: 4-4-Bravo-3-0.
Klaatu: What size is that suit?
Polygraph Operator: 42 long.
Klaatu: Take it off.
Klaatu escapes and makes it as far as a train station when his bullet wound opens and he needs Dr. Benson's help. Helen is at home with Jacob.
Helen: Have you done your homework?
Jacob: School's canceled on account of the aliens. They say it's an invasion.
Jacob is watching the news showing the presence of the sphere has caused a worldwide panic. Helen gets a call from the train station that Klaatu is there asking for her and she rushes over there with Jacob in the back seat.
Helen had taken a sample of Klaatu's placenta earlier and he is aware of it. Klaatu requests the sample and rubs a dab of it on his wound. It heals within seconds. After leaving the train station, Klaatu gives her directions to a specific location.