John Smith is an alien from the planet Lorien. He was sent to Earth as a child with eight others to escape the invading Mogadorians, who destroyed Lorien. Here, John is protected by a Guardian, Henri, and has developed powers, including enhanced strength, speed and agility, as well as seven other powers he will develop later in life, known as legacies. He is protected by a Loric charm which protects him from all Mogadorian harm as long as he is hurt out of order. He can only be hurt after the first three loriens are killed in order.
The Mogadorians, led by the Commander, learn about the nine children and come to Earth to find them. The Lorien can only be killed in sequence; Number One through Number Nine. Three of them are already dead, with John being Number Four. Knowing this, he and Henri move from a beachside bungalow in Florida to an old farm in Paradise, Ohio, where John befriends conspiracy theorist Sam Goode and a dog which he names after Bernie Kosar. He also falls for an amateur photographer, Sarah Hart. Her ex-boyfriend, football player Mark James is a bully who torments both John and Sam.
During the Halloween Festival, Mark and his friends chase Sarah and John into the woods, where they try to beat John up. However, he uses his powers to fend them off and rescue Sarah. Sam witnesses this, which forces John to reveal his true origins. The next day, Mark’s father, the local sheriff, interrogates Henri on John’s whereabouts when his son and his friends were attacked.
Henri tells John that too many people are suspicious of them so they have to leave. John refuses because of Sarah. Meanwhile, the Mogadorians continue searching for John, while being trailed by another Garde, Number Six, who is also trying to locate Number Four. Number Six’s guardian was killed, and she realizes that the remaining six of the Garde will have to team up and fight against the Mogadorians. She knows Number Three is dead and that Number Four is being hunted.