In the gripping opening scene, we find Cassie Sullivan (Chloë Grace Moretz) navigating through the dense woods, armed with a gun. She stumbles across an empty store and scavenges for supplies, when suddenly a faint cry for help catches her attention. Cassie discovers a man hiding in the back, and tension escalates as both aim their weapons at each other. The man slowly lowers his gun, but Cassie remains wary, insisting he show his hands. As he reveals a crucifix, a flash of light surprises her, leading to a shocking moment where Cassie reacts by shooting him, horrified at her own actions when she realizes the significance of what he held.
Months prior, Cassie was just an ordinary 15-year-old enjoying life in a suburb of Cleveland, Ohio, with her parents, Oliver (Ron Livingston) and Lisa (Maggie Siff), along with her younger brother, Sam (Zackary Arthur). She cherished her friendship with Lizbeth (Gabriela Lopez) and had a crush on the school’s football star, Ben Parish (Nick Robinson). However, everything took a dramatic turn when ominous alien spaceships descended from the sky, looming above humanity and instilling fear for days. These extraterrestrial beings would soon be known as "The Others."
The first wave initiated by these invaders unleashed an electromagnetic pulse, bringing a catastrophic halt to all electronic power on Earth, causing planes to crash and claiming countless lives. Struggling to survive, Cassie and Sam are forced to retrieve water from a nearby lake due to the lack of a clean water supply.
The second wave brought about catastrophic earthquakes and devastating tsunamis, annihilating coastal cities and taking a further toll on humanity. Following this, the third wave saw the modification of the avian flu by the Others, resulting in infections spreading among the population, including Lizbeth and even Cassie's mother, Lisa. In a heart-wrenching moment, Cassie, Sam, and Oliver lay Lisa to rest in their front yard and bid her a tearful farewell.
As they seek refuge in a survivors' camp, military forces, led by Colonel Vosch (Liev Schreiber), arrive claiming to offer assistance. They gather the children and take them to safety, a decision that weighs heavily on Oliver as he allows Cassie and Sam to go. However, when Sam forgets his cherished teddy bear, Cassie rushes off the bus to retrieve it, only to watch helplessly as the vehicles depart and leave her behind.
With the fourth wave underway, Colonel Vosch reveals that the Others have begun using humans as hosts, sparking fear and panic among the adults, which leads to a deadly confrontation where nearly all the adults, including Oliver, are senselessly killed. Cassie grieves her father's loss and embarks on a perilous journey alone.
The children, now at Camp Haven, undergo military training, where Ben and Sam receive new code names—Ben becomes "Zombie" and Sam "Nugget." Ben encounters Sergeant Reznik (Maria Bello), who prepares him for the unsettling truths of their reality, showing him a boy believed to be an Other and forcing him into a moral quandary that pushes him to a breaking point.
Navigating through the desolate ruins of their former lives, Cassie arrives at the same convenience store from the beginning, where she faces yet another life-or-death scenario. Meanwhile, Ben trains alongside his peers and bonds with Dumbo (Tony Revolori) and Ringer (Maika Monroe), a fierce warrior who empowers them to confront the challenges ahead.
After enduring injuries and trauma herself—Cassie finds a new ally in Evan Walker (Alex Roe), a mysterious man who treats her wounds but harbors secrets about his true nature. As Cassie and Evan grow closer, they face other dangers, including the deceptive Lieutenant Colonel Vosch, who manipulates their lives behind the scenes.
The stakes rise as Cassie and the others uncover the horrifying truth: they themselves have been unwitting pawns in the Others' strategy, thus intensifying their resistance against the alien forces. With alliances forming and courage tested, Cassie must confront the unsettling reality about Evan’s identity and her own survival.
In the gripping climax, friendships are forged in the chaos of battle, and sacrifices made resonate deeply as Cassie and her allies strive for freedom against the shadows of despair. As hope flickers, the film closes with Cassie reflecting on their resilience and the enduring flame of hope that propels them forward, reminding audiences that even in the darkest times, the light of humanity shines through.
