An alien from the distant planet Lorien, John, portrayed by Alex Pettyfer, was sent to Earth as a child alongside eight others to escape the menacing Mogadorians, who annihilated their home world. He comes under the protection of a guardian named Henri, played by Timothy Olyphant, and over time, John discovers he possesses remarkable abilities attributed to his alien heritage, including enhanced strength, agility, speed, telekinesis, fire resistance, and the unique ability to emit light from his hands.
Trouble arises when the Mogadorians, led by a ruthless Commander (Kevin Durand), uncover the existence of the nine survivors and arrive on Earth determined to hunt them down. They can only be defeated in a specific order, having already taken the lives of three. John identifies himself as Number Four, and he and Henri decide to relocate from Florida to the quiet town of Paradise, Ohio. Here, John befriends Sam Goode (Callan McAuliffe), an enthusiastic conspiracy theorist, and develops a romantic connection with Sarah Hart, an aspiring photographer and classmate played by Dianna Agron. However, their happiness is threatened by Sarah's ex-boyfriend, Mark James (Jake Abel), whose bullying behaviors create tension.
Amidst the chaos, Henri safeguards a mysterious box given to John by his father as they fled Lorien, a box he cherishes and promises to give to John when the moment is right. Henri reveals to John that he was specifically chosen by the Loriens to inherit extraordinary powers from his parents, crafted for a significant destiny. Sam, driven by a desire to uncover the truth about his missing father—the former steel plant worker who sought to construct a platform for alien vessels—shares his insights, revealing Henri's discreet investigations into the conspiracy surrounding his father.
During the vibrant spring carnival, Mark and his friends capture John and Sarah, wrongly believing they are involved romantically, and they pursue them into the woods where they attempt to assault John. He uses his powers to protect both himself and Sarah, an act witnessed by Sam, who finally learns John's true origin. Soon after, Mark's father, the local sheriff, confronts Henri, scrutinizing John's involvement during the attack on his son and friends. The duo must contemplate fleeing as scrutiny mounts; yet, John is unwilling to leave Sarah behind, leading to a conflict with Henri, who has his own burdens trying to prepare John for the looming confrontation with the Mogadorians.
As the Mogadorians continue their search, another survivor, Number Six (also played by Jake Abel), resolves to take a stand against them after her guardian is slain. The Mogadorians capture Henri using deceitful tactics, pushing John and Sam into action in an attempt to save him. Although they manage to escape, Henri suffers fatal injuries during the struggle, imparting to John the importance of their mission before dying, alongside a collection of Lorien artifacts and a crucial blue rock that serves as a locator for the remaining Loriens.
As John wrestles with heartache after his farewell with Sarah, the Mogadorians frame him and Henri, branding them as terrorists. With drama escalating, Mark inadvertently aids in revealing John's powers while saving Sarah, igniting their escape to safety at their high school. Meanwhile, the Commander takes direct action by sealing off the town with his forces, seeking revenge on John.
During a climactic confrontation, John, Sarah, and Sam face down the Commander and his monstrous henchmen. Their fortunes are turned when Number Six and John's faithful companion, a shapeshifting creature known as Bernie Kosar, intervene. Bernie reveals his extraordinary capabilities before transforming back into his beagle form after the battle. Together, John and Number Six combine their powers, successfully defeating the Mogadorians, culminating in an explosive end for the Commander.
The following morning, united by their determination, John, Number Six, Sam, and Bernie Kosar channel their collected blue rocks to reveal the whereabouts of the other four remaining Loriens. They embark on a mission to gather their kind and prepare to defend Earth against the Mogadorian threat, while leaving behind Sarah—and a reformed Mark, who bravely lies to his father about John's location, ultimately returning to John a box that once belonged to his father.
