At a lively party, teenager Courtney is persuaded by her friends to download a mysterious app called Countdown, which claims to predict how much time a user has left to live. To her shock, the app reveals that she has merely three hours remaining before her time expires. When she wisely avoids getting into a car with her inebriated boyfriend Evan, she receives a chilling notification stating that she has violated the "user agreement." Tragically, upon returning home, she's attacked by an unseen force and faces a grim fate as her countdown hits zero. Simultaneously, Evan encounters a catastrophic accident, with a tree branch crashing through his seat – the very spot where Courtney would have been.
Afterwards, Quinn Harris, a nurse at the hospital where Evan arrives injured, faces her own set of challenges. She's a target of continuous harassment from her boss, Dr. Sullivan, and is too fearful to speak out since she has only just achieved her nursing credentials. Upon discovering Evan in a restricted area of the hospital, he reveals his deepening fears related to Courtney and the ominous app. Though she initially dismisses his fears, Quinn succumbs to peer pressure and downloads Countdown, only to be horrified when it warns her that she has just three days left to live. After Evan decides to skip his surgery, he unwittingly breaks the app's user agreement, leading to a chilling encounter with a demonic version of Courtney that results in his death.
In the wake of this chaos, Quinn visits her home to retrieve a crucial document and faces her younger sister, Jordan, whose relationship with Quinn has been strained since their mother passed away. The next day, upon learning of Evan's death, she snatches up his phone. After canceling plans to spend time with Jordan and their father, she receives yet another shocking notification indicating that she has also broken the user agreement. As she endures further harassment from Sullivan, she embarks on a frantic search for answers about the app, learning about Courtney’s demise and others who faced similar fates—though many dismiss these as hoaxes. Terrified by a vision of the deceased Evan, Quinn abandons her apartment.
The following day, Jordan finds Quinn and uncovers the horrifying news: the app indicates Quinn has only a day left to live. In a desperate attempt to escape her fate, Quinn purchases a new phone, but to her horror, the Countdown app installs itself again. After a terrifying encounter with a demonic figure, she meets Matt, whose app predicts his death just hours before Quinn's.
Together, they turn to the hospital's priest, Father John, for help, only to discover that Quinn has been falsely accused by Sullivan of sexual harassment. This leads to her suspension, while Matt faces an assault by the app’s sinister entity, which has taken the form of his deceased brother. Join forces, they discover that the user agreement can be circumvented if they reject their predetermined deaths. With this newfound hope, they seek Father John's guidance, learning that the app’s coding may hold the key to their survival. They enlist the help of Derek, a phone salesman, and uncover the dreadful truth: Jordan has unwittingly downloaded the app, sealing her fate just before Quinn's predicted demise.
In a frantic race against time, they modify the app to extend their lifespans, but their victory is short-lived when Quinn is attacked, and their countdowns reset. After Jordan is viciously assaulted by the Ozhin, a demon identified by Father John, they devise a last-ditch plan to survive: they must beat the countdown by one second. Despite preparing a protective warding circle, Matt is lured outside by the Ozhin and ultimately meets his tragic end.
When Jordan is also targeted by the demon, Quinn, driven by desperation, makes the ultimate sacrifice by overdosing, intending to take down the Ozhin with her. In a moment of clarity, she scribbles "Narcan" on her arm in a bid for survival. Thankfully, Jordan revives her with Naloxone, and they manage to stop the timers just in time.
Later, the sisters and their father pay a visit to their mother's grave. As they leave, they hear about Sullivan's arrest following multiple allegations from fellow nurses about his misconduct. Just when they think they are safe, Quinn discovers that her phone has ominously updated to Countdown 2.0.
In a chilling mid-credit scene, the phone salesman Derek finds himself on a date, only for his Countdown app to alert him that he has broken the user agreement, with dreadful consequences lurking just around the corner.
