Plot:
In his New York City apartment, a young boy named Josh Morrison (Austyn Lind Myers) stares through his telescope at an object falling from the sky. It is a golf-ball-sized metal ball which flies through the window and lands in his fishbowl, quickly draining the water along with the goldfish. He decides to show it at his school's science class presentation.
Some months later a massive fireball crashes into the water near Liberty Island. It is revealed to be a spaceship which resembles a human (Eddie Murphy), controlled by 100 tiny humanoid aliens. Its Captain (also played by Murphy) pilots the spaceship from the command deck located in its head, with the help of his second-in-command Number 2 (Ed Helms), and researcher Number 3 (Gabrielle Union). The spaceship looks very human, but the aliens don't know how to act in society and the ship displays numerous superpowers. A superstitious cop named Dooley (Scott Caan) desperately searches for the alien.
The aliens need to save their planet, Nil, from an energy crisis. They need salt, which they plan to take by draining the Earth's oceans using the metal ball, so they have to recover it. After the spaceship is hit by Josh's single mother Gina Morrison (Elizabeth Banks) and her car, the Captain decides to befriend Gina and Josh, telling them his name is Dave Ming Cheng, based on a quick scan of common Earth names, and Gina's comment that he looks more like a Dave than a Ming Cheng. They then see their missing ball in a photograph taken at the science presentation. After having breakfast with Gina, "Dave" goes to Josh's school where he pretends to be a substitute teacher and eventually is able to talk to Josh alone. Josh tells him that the ball was taken from him by a bully. With Josh's help, Dave takes the metal ball back from the bully.
The Captain (via Dave) spends some time with Josh and Gina and realizes that humans are more advanced than they thought, having feelings and love, such as witnessing Gina's painting or a homeless man offering to share his blanket with Dave when he sleeps in a doorway. He decides to cancel the plan to drain the oceans because it would destroy life on Earth. The police track Dave down using the impression of his face found in the dirt at the crash site and they arrest him. After spending too much time on Earth, most of the crew begins to exhibit new "feelings". Number 2 decides that the captain and the rest of the crew's changing behavior is unacceptable so he takes command and imprisons the Captain. Under Number 2's command, Dave breaks out of the police station and they try to arrest him again. Number 3, who has become infatuated with the Captain, becomes jealous of Gina. She first cooperates in the command change but later agrees with the Captain's view on humans. Both are caught by Number 2, and they are expelled from the spaceship. In the meantime, Number 17 (Kevin Hart), a young, fun-loving alien jumps out while drunk from the alcohol Dave has imbibed. The Captain apologizes to Number 3 for ignoring her. He admits that he too loves her and wants to be with her. Back at the police station, Dooley discovers Number 17 in his coffee and interrogates him to find out where Dave is going.
Number 2 takes Dave to the harbor, where he tries to throw the metal orb into the ocean, but is stopped by the Captain and Number 3, who managed to gain reentry back onto the ship. They convince the rest of the crew that the real Captain is in charge again. Reinstated, he orders Number 2 to be stuck in the ship's "butt" forever. The metal orb meanwhile slips out of Dave's hand and rolls into the ocean. The Captain attempts to retrieve the orb but is told that they only have enough power to either retrieve it or return home. The Captain decides to save the Earth and the rest of the crew agrees. The ball, thrown in the ocean by Number 2, is retrieved. Dave powers down while Dooley and his partner catch up and point their guns at him. With no power, Dave's shields are disabled, leaving the crew defenseless. Josh tries to tell the police officers that Dave is harmless but is ignored. He then grabs Dooley's taser which he uses on Dave, recharging him. The captain and Number 3 reveal themselves to the police officers who stand down. The Captain says goodbye to Josh and Gina saying he now understands love. Number 17 is allowed to enter Dave. About to fly away, a team from the FBI arrives and throws a net over Dave. While the FBI agents wrestle the body down, "Dave's" crew evacuates to one of the ship's "lifeboat" shoes, activate the engines, detaches the shoe and heads home to Nil, leaving behind both the ship and Number 2. While in the lifeboat, The captain asks for Number 3's hand in marriage. She accepts and they kiss.