Los Angeles Police Department veteran 9-1-1 operator Jordan Turner receives a call one night from a teenage girl, Leah Templeton, fearing for her life as a man breaks into her home. Jordan calmly advises her to conceal herself upstairs. When the call is disconnected, Jordan calls Leah back, a decision that costs Leah her life. The ringing phone gives her location away to the intruder. Jordan attempts to dissuade him from going further over the phone. He responds “It’s already done” and hangs up, leaving her distraught.
The next day, Jordan sees a television report confirming that Leah has been murdered. Emotionally affected by the incident, Jordan tells her boyfriend, Officer Paul Phillips, that she can no longer handle field calls.
Six months later, she is working as a trainer for 9-1-1 operators. Teenager Casey Welson is kidnapped from a mall parking garage by a man named Michael Foster after shopping with her friend, Autumn, and forced into the trunk of his car, a red Toyota Camry LE. Casey had taken Autumn’s disposable phone after she accidentally left it in the mall, and uses it to call 9-1-1. A rookie operator, Brooke, receives the call but is unable to handle it, prompting Jordan to take over. However, Casey’s use of a disposable Phase I phone means that her location cannot be determined by GPS, showing only the location of the Cell Tower the disposable is connected to.
While Michael drives, Jordan guides Casey through knocking out a tail light and signaling people in nearby cars. One woman who sees Casey waving her arm out of the trunk of Camry calls 9-1-1 on a phone with Phase II capabilities, allowing the police to narrow their search. When Casey tells Jordan that there are cans of paint in the trunk, Jordan has Casey pour the paint out of the taillight hole. This attracts the attention of another motorist, Alan Denado, but Michael knocks him out with a shovel, and assuming him to be dead stuffs Alan in the trunk of a black Lincoln Town Car Signature Limited with Casey.
Alan reawakens in the trunk and begins screaming uncontrollably, and Michael stabs him, unfortunately Alan gives away an Escape Lever to Michael who rips it out. Michael stops at a gas station where Casey gets an idea to crawl out of the trunk and scream for help. A gas attendant attempts to bust the door open, but Michael violently douses him with gasoline and throws a lighter at him, burning him alive.
Upon arriving at his destination, Michael removes Casey from the trunk and finds the phone in her pocket, with 9-1-1 listening on the line. Through the phone, Jordan informs him that his identity is uncovered and advises him to surrender himself and not hurt the girl. Before smashing the phone, Michael responds, “It’s already done”, leading Jordan to realize that Michael is the same culprit who killed Leah six months prior.
Meanwhile, Paul, accompanied by Officer Jake Devans and others in law enforcement, raid Michael’s home, but find only his wife and children. Finding a photo of Michael and his sister, Melinda, they realize that Casey resembles her. Additionally, the house seen in the photo is eventually revealed by Michael’s wife to have burned down, although a nearby cottage still remains. The police raid it, but find nobody there and leave.
Michael begins to process and torture Casey slowly. Determined to rescue Casey, Jordan drives to the secondary home where she finds a number of photos of Michael with his leukemia-stricken sister. Stepping outside, she recognizes sounds from an outdoor flagpole—exactly the same sound she heard in the background in the final moments of the 9-1-1 call. She also finds a trap door amid dirt and scrub located where the primary house once stood. Jordan accidentally drops her cell phone down the cellar and climbs down in without calling the police.
Navigating the cellar, Jordan hides from an emerging Michael. The cellar itself pieces together Michael’s backstory. Michael had incestuous feelings towards Melinda and was distraught when she fell ill and died. Michael has a prop head that he keeps in the cellar that he treats like his sister. He has also been scalping and killing young girls who have similar blond hair, trying to find scalps that match his sister’s hair, which she lost due to chemotherapy.
Jordan soon finds Casey strapped down to a foldable wheelchair and attacks Michael, as he begins to lacerate Casey’s scalp. She frees Casey and both make an attempt to escape the cellar, during which Casey stabs Michael in the back with a pair of scissors. Michael pursues them and they are able to injure and kick him back down into the cellar, rendering him unconscious. While he is knocked out, they tie him to the same chair Casey was restrained to during captivity.
When he regains consciousness, they inform him that they plan to leave him to die and claim that Casey had escaped, that she was found by Jordan in the woods, and that Michael disappeared, though Michael was actually present at that time. Michael pleads that they cannot just leave him there, but Jordan replies using Michael’s own words, “It’s already done.” Michael shouts insults at them, Jordan locks the door and leaves, avenging Leah’s death.