John Smith is an alien from the planet Lorien. He was sent to Earth as a child with eight others, collectively called the Garde, to escape the invading Mogadorians, who destroyed Lorien. John is protected by a guardian (who are known as C�pan), Henri, and has developed powers, including enhanced strength, speed and agility.
The Mogadorians, led by the Commander, learn about the nine children and travel to Earth. The Garde 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 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. When they attack, John uses his powers to fend them off and rescue Sarah. Sam witnesses this, which leads 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 and they have to leave. John refuses because of Sarah.
The Mogadorians continue searching for John, while being trailed by Number Six, who is also trying to locate Number Four. Number Six's guardian was killed, and she realizes that the remaining six Garde will have to team up and fight against the Mogadorians.
The Mogadorians locate John and manipulate two conspiracy theorists into capturing Henri. When John and Sam go to rescue Henri, they have to fend off an attack by the Mogadorians. Henri dies, while John and Sam escape with some Lorien artefacts, including a blue rock that acts as a tracking device for other Garde. Sam's father, a conspiracy theorist who disappeared while hunting aliens in Mexico, had another of the rocks. While Sam searches for it, John tries to say goodbye to Sarah at a party, only to discover that the Mogadorians have framed him and Henri for the murders of the conspiracy theorists. Mark sees John and calls his father, who corners John and Sarah. John saves Sarah from a fall, revealing his powers, and they escape to their school.
The Commander arrives in Paradise, with a convoy of trucks. He is confronted by Mark and his father, and after injuring the sheriff, he forces Mark to show him where John is hiding, which Mark has deduced is the school.
John, Sarah, and Sam are attacked by the Mogadorians, who brought two Piken to hunt the trio. They are saved by Number Six and Bernie, who is actually a shapeshifting Chimera sent by John's biological parents to protect him. John and Number Six, who can turn invisible and can block energy attacks, continue to fight the Mogadorians. They eventually defeat them all, including the Commander.
The following day, John and Number Six unite their blue rocks and discover the location of the other four surviving Garde. John lets Sam come with them in hopes of finding Sam's father. They set off to find the others so they can protect Earth from the Mogadorians, leaving Sarah and a repentant Mark, who lies to his father about John's whereabouts. Mark also returns a magic box left to John by his Dad, that was in police evidence. John thanks Mark and promises Sarah that he will come back to her. While they share a goodbye kiss, Mark is visibly hurt and agitated. John, Sam, Bernie and Number Six drive off, vowing to protect Earth.
