John Smith was sent to Earth as a child along with 8 other children when their home planet was destoryed by the Mogadorians. However, the Mogadorians soon find out that these children were sent to Earth and travel there to destroy them. The children can only be killed in sequence to their numbers and three are dead. John just happens to be number four and is next on the hit list.
John has a guardian named Henry who helps him understand his origins, and the legacies that he gains as he gets older. Legacies are like superhuman powers such as increased agility and telekinesis. John and Henry decide to move to a new home to better their chances at survival when they realize that they are next on the hit list.
At his new school John makes a new friend named Sam and falls for a girl named Sarah. Later on he finds a dog and names the animal Bernie Kosa. At a Halloween Festival John uses his legacies to protect and save Sarah and Sam witnesses it happening. To keep Sam as his friend and to protect himself he explains his situation to Sam who agrees to keep the secret.
Soon after this number six decides to hunt down number four after her Guardian is killed. She knows that if any of the remaining nine have any chance at survival they must team up and fight against the Mogadorians. She also knows that number three is dead and that number four is currently being hunted.
Later on Henry is killed saving Sam and John from a conspiracy theorist who is hired to capture John. When they escape they take with them a Lorien artifact that can help them track down the other numbers. In the end there is a big fight at the highschool with the mogadorians and number six joins the fight. They win the fight and continue with Sam to find the others and stand up to the Mogadorians.