A teenage boy named John Smith leads an unusual life, constantly moving from town to town with his guardian, Henri, to evade mysterious enemies. John is one of nine alien children who fled to Earth after their planet, Lorien, was destroyed by the deadly Mogadorians. Each of these children possesses unique abilities, referred to as Legacies. John is hiding his true identity and blending in as a normal high school student in Paradise, Ohio, but finds it increasingly difficult as his powers begin to manifest, attracting unwanted attention. As he tries to navigate teenage life, he befriends Sarah, a compassionate photographer, and Sam, an alien conspiracy theorist.

John learns more about his origins and the fate that awaits him while trying to prepare for an inevitable confrontation with the Mogadorians, who have already killed three of his fellow Loriens. As the threats grow closer, Henri and John struggle to stay one step ahead and protect each other. While adapting to his growing powers, John must decide whom he can trust, risking exposure and endangering those he cares about. The situation escalates as relentless pursuers close in, leaving John to face a critical choice that will determine the safety of not just himself, but the future of all the Loriens.