Clay Riddell has just landed at the Boston airport and eagerly contacts his wife, Sharon, sharing his excitement about securing a deal for his graphic novel and its adaptations. While she expresses happiness, an unexpected twist arises when she reminds him that he had left a year ago, leading to a strained moment. Despite their fractured relationship, Clay yearns to connect with his son, Johnny. However, their conversation abruptly ends when his phone dies, prompting him to search helplessly for a charger. Facing frustration, Clay resorts to using a pay phone to call Sharon again.
In a shocking turn of events, chaos erupts as everyone using a cell phone responds to a dissonant sound, leading to violent and catastrophic behavior. The situation spirals, resulting in an airplane crash and overwhelming police distress. Amid the turmoil, Clay narrowly escapes via a subway entrance only to find a stalled train alongside a conductor named Tom and several others. With no power, they decide to trek down the dark tracks, facing grotesque, zombie-like figures dubbed "phoners." After a harrowing confrontation, Clay and Tom flee through a manhole, revealing a Boston transformed into a war zone, filled with wrecked cars and flames.
When they finally reach Clay’s apartment, a horrifying incident unfolds outside involving a motorcycle rider and a pack of phoners. Inside, Clay remembers his unsettling artwork, which eerily resembles the movie's antagonist. Desperate to reconnect, Clay contemplates calling Sharon, but Tom warns him of the continuing danger, while text messages yield nonsensical replies. Their isolation is interrupted by a bloodied neighbor, Alice, who pleads for refuge after witnessing her mother’s murder.
The trio, now uneasy allies, explore the outside world, uncovering a grim scene near a cell tower. Inside a nearby house, they find deceased individuals clutching firearms. Tom, Clay, and Alice loot what they can, only to encounter another stunned individual who morphs into a phoner before their eyes, leading to further confrontation. As noise emanates from the phoners, it becomes apparent that they follow a collective instinct, suggesting they operate like a superorganism.
Under the cover of night, they reach Gaitan Academy, where only Headmaster Charles Adai and a student named Jordan remain. They receive a warm welcome but discover a chilling sight: a sea of phoners resting across the school's soccer field. Charles posits that the phoners may represent a new evolutionary stage, prompting a debate among the group about the moral implications of this transformation. Determined to fight back, they attempt to ignite a fire on the field, leading to explosive chaos that claims Charles’s life.
As snowfall blankets the area, they seek shelter in a drive-in theater. In a poignant moment, Tom shares a scripture-laden sermon reflecting on sin, and Clay faces unsettling dreams reminiscent of his artwork. With each passing day, the group finds temporary solace in a restaurant with a "No Fo" sign, celebrating with music and dance, while grappling with their pasts.
Tragedy soon strikes when a member, Sally, transforms into a phoner after receiving a signal, igniting chaos within the restaurant. In the chaos, Alice suffers a fatal injury, leaving Clay and the remaining members to press on. Their next encounter leads them to a camping ground where they meet Ray and Denise. A survivalist at heart, Ray warns them of impending dangers posed by the phoners and hints at a deeper connection to the enigma surrounding the "man in red."
The extremities of survival push Clay to confront deeply personal conflicts, culminating in devastating decisions regarding the fate of his loved ones. After experiencing profound loss, he embarks on a perilous journey back to Kashwak, unveiling a colossal tower with throngs of phoners surrounding it. In a climactic showdown against the man in red, Clay fights to disentangle his son from the clutches of the phoner hive mind. In a moment of ultimate sacrifice, he detonates an explosive truck, liberating the phoners and reclaiming his son’s humanity, leading them toward a glimmer of hope in a fractured world.
