The story follows nine alien children from the planet Lorien who escape to Earth to hide from their enemies, the Mogadorians. After Numbers 1, 2, and 3 are killed, the next target becomes Number 4, who lives on Earth under the name John Smith with his guardian Henri. As they move from place to place to stay hidden, John begins to develop special powers. When he joins a new high school, he forms a relationship with a girl named Sarah and befriends a boy named Sam, who helps him uncover the truth about his identity. Eventually, the Mogadorians track him down, leading to a major battle where John teams up with Number 6, another Lorien survivor. In the end, John survives the fight and decides to continue searching for the remaining members of his kind.