Nine infant aliens, who closely resemble humans, flee their home planet, Lorien, to hide on Earth. An invading species, the Mogadorians, have destroyed their planet, and follow them to Earth to hunt them down. Each of the nine aliens is given a guardian and will develop superhuman powers as they become adults. They are each assigned a number. These last children of Lorien can only be killed in the sequence of their numbers. Numbers One, Two, and Three have been killed so far.
Number Four (Alex Pettyfer), also named John Smith, moves to Paradise, Ohio, disguised as an American high school student. He makes a friend, Sarah Hart (Dianna Agron), a sweet Midwestern girl who is a photographer. After being on the run his whole life, Number Four falls in love and now has something to stand up and fight for.