It’s twilight, and a teenage couple is “parking" on an overlook of their town. Suddenly, we see a spaceship fly by; they drive off in a panic.
Morning comes, and the town is going about their regular routine: kids going to school, parents going to work, and a lady is walking her “dog” on his leash. As he pees, it burns through metal, and we see that it is an alien-type dog. In fact, everyone in the town is green-skinned, and look like sea monkeys. This alien planet has lives like our 1950’s. It begins to rain, and they pull out umbrellas, but their rain is small rocks instead of water.
At the Planetarium, Lem (Justin Long) is giving a lecture to a group of grade-schoolers, and tries to interest them in space. He tells them that they know lots of things about space, like their universe is 500 miles long.
Lem’s friend, Skiff (Sean William Scott), is excited for the premiere of “Humaniacs III”, a science-fiction sequel about hu-maniacs with one giant eyeball who attack, and want to consume their brains and take over their planet, in just 2 days. So is his little brother, Eckle (Freddie Benedict). Lem thinks it is silly and not too interested.
A small “Rover”, sent up by Earth years ago, senses that an American spaceship is nearby, and breaks out of his containment in the army base. When the army soldiers realize he escaped, they think that the battle to save their world has begun. He follows the signal to the area near Lem’s house.
At Lem’s house, the fish in the bowl act crazy, the radio station goes out and the spaceship lands in Lem’s backyard. We see an astronaut climb out, holding a large USA flag, and plants it in the ground. He turns around and see aliens barbequing, panics, screams, and runs back to his ship.
The army sends troops and tanks to find the spaceship and “alien” and he hides in the planetarium. When Lem goes to work, he finds the astronaut. He takes off his helmet and is surprised he can breathe. He introduces himself as Captain Chuck Baker (Dwayne Johnson). Earlier Rover photos showed what he thought was an uninhabited planet, and he was sent to plant the flag, take some photos and samples and go back. But now, with the army looking for him, he can’t get to his spaceship. He must return to his big ship, which will only orbit for 74 more hours before heading back to Earth. Lem agrees to help.
Neera (Jessica Biel) and Glar, friends of Lem’s are outside. They think the military is over-reacting, and that the spaceship might have come in peace. Lem won’t join their protest as he has to help Chuck. Neera is disappointed in Lem. Lem has a big crush on her.Lem and Chuck hide out in Lem’s bedroom. Eckle and Skiff come by. Rover sneaks in through the doggie door. Before they can sneak out, 2 army troops come in; they see the “alien” and think he can control their minds. So, Chuck pretends to use mind control, and gets them to take them in their vehicle. Lem thinks they need to get to the reporter covering the story, so that Chuck can address the town and let them know he isn’t dangerous. On the way, Lem sees Neera, and she tells him that she needs someone that doesn’t always believe everything he is told, and she moves closer to Glar. Lem is crushed. He takes Chuck to hide again, back at the Planetarium. He thinks his life is over, because he got fired, and Neera gave up on him. Chuck consoles him, and tells him he has “the right stuff” because he risked everything to help him.
Before they can do an on-air interview with Chuck, the troops take Chuck’s spaceship back to the secret hiding place, “Base 9”, to take it apart and study it. The Professor Kipple (John Cleese) there wants to dissect Chuck and study his brain.
To escape and get to Base 9 and the spaceship, the groups disguise themselves in “Humaniac” costumes, and blend in with the costume contest at the movie premiere. The General (Gary Oldman) sees the flag on Chuck’s spacesuit, and knows it is him. The troops catch Chuck and Rover, and take them to Base 9.
Lem, Neera, Glar, Eckle and Skiff drive to Base 9, with Rover using his tracking device to show the way. It is hidden in the desert, and when Skiff takes a soda, it opens the entrance to the base, which is a huge underground operation.
To get in the base, the group pretend to be hippies, singing Bob Dylan-esque songs, and then Rover takes off to find Chuck and the spaceship. Rover finds the spaceship, and Eckle pushes a red button, which opens the launch hatch. They find Chuck, just before Professor Kipple is about to slice open his brain. They race to the spaceship, but it is a trapthe General and troops are there waiting. Lem explains to the General that he shouldn’t be afraid of the unknown, and that we need to learn new things. The Base 9 self-destruct button is pushed, and the countdown ‘til explosion begins, in 2 minutes. The group boards the spaceship to escape, and then Chuck sees the General has been knocked out, and left on the ground. He leaves the ship to go save the General, and brings him on the ship. The ship takes off Justin time, before the Base blows up. They fly in space, and Lem, Neera, Eckle and Skiff are amazed to see large planets and stars, and that the universe is huge, much larger than the 500 miles that they’d been taught. Chuck flies them back to town, and drops them off at home. Rover wants to stay with Skiff. Lem starts to ask Neera out, and before he can finish, she kisses him. Lem and Chuck shake hands, and share a “man hug” which is something they do on Earth, he tells everyone.
Chuck takes off, and we see in the window that the alien dog has snuck in the spaceship with Chuck, headed back to Earth.