Plot
The film opens at a bustling carnival at night. Amid the noise and flashing lights, the scene cuts to a nearby house where Stacy, a teenage babysitter, begins receiving phone calls filled only with heavy breathing. A light suddenly flicks on, revealing a figure rushing toward her. Stacy's scream blends with the carnival crowd, masking the attack. Police later arrive to find Stacy and the three children she was watching murdered.
Meanwhile, in Colorado, 125 miles away from the crime scene, Jill Johnson runs laps on the indoor track above the basketball court. Later, Jill talks with her friend Scarlett about missing the school's bonfire, having been grounded for going over her cell-phone minutes while arguing with her ex about kissing her other friend, Tiffany. She has to babysit that night to help pay off the bill.
That evening, Jill's father drives her through winding mountain roads to the Mandrakis residence, a secluded modern glass house on a lake. Along the way, he tells her he and her mother are going to a classical concert, and that Mr. Mandrakis will bring her home afterward. At the house, Mr. and Mrs. Mandrakis greet her warmly and give a short tour, mentioning that their two children are ill with the flu and already asleep upstairs. They say they'll return around midnight, then leave.
Jill tries to pass the time in the quiet house. Growing bored, she wanders into the master bedroom, tries on a few pieces of Mrs. Mandrakis's jewelry, and is startled when she hears a noise. Following the sound, she discovers Rosa, the live-in maid, feeding birds in the indoor atrium. Back in the living room, the phone rings. Jill answers but hears only breathing before the line disconnects. Later, she calls Scarlett; she mentions Bobby asking for Jill's number, and Jill tells her to give it to him before hanging up.
Moments after hanging up, the security alarm blares, echoing through the house. Jill disarms it and thinking Rosa triggered it, she rushes to the atrium—but finds only the chair with bird feed, and the birds still eating. As Jill heads back toward the living room, the phone rings again. She answers; it's silent at first, then a calm male voice asks, "Is everything alright?" Confused, Jill asks if it's Doctor Mandrakis but the caller hangs up.
Later, the phone rings again, and this time it's a group of boys from school, laughing before hanging up. Moments later, she hears another strange noise from the kitchen. She moves cautiously to check—only to discover it's just the ice machine rattling as it dispenses cubes. She grabs a popsicle and as she takes a bite the phone rings again. She answers it and after a brief silence she hears a voice behind her say "Hello"; it is revealed to be Tiffany, who was sent by Scarlett to make up with Jill. Jill asks how she got in, and Tiffany says the garage was open; this worries Jill; she rushes to close it. The two make small talk briefly then make up, and Tiffany leaves for the bonfire.
Outside, Tiffany hears footsteps; she drops her keys then retrieves them and rushes to unlock the car door. After getting inside, she sees a branch blocking the driveway and reluctantly gets out to move it. The camera rushes toward her from behind but we don't see what happens. Inside, Jill is tossing away her melted popsicle when loud banging hits the front door. Thinking it's Tiffany, she opens it—but no one's there. Uneasy, she closes the door and resets the alarm before sitting back down. Jill receives another call—this time from her ex, Bobby, who admits their friend Cody prank-called her earlier but insists it was only once. Jill asks if he's sure, but the call is dropped from poor signal.
Soon after, the phone rings again. Caller ID shows Tiffany, but when Jill answers, a man's voice says it's not. When she asks if it's Cody, he asks "Who is that?" and hangs up. Uneasy, Jill tries calling Scarlett, but the call drops after a few seconds. She dials her father's number, then the Mandrakises, and leaves voicemails for both, asking them to call her back.
Growing more anxious, Jill contacts the non-emergency police line, explaining that she's receiving disturbing calls. She is told they can't intervene unless an actual threat is made. As she's speaking, Jill notices Rosa's car parked outside. She tells the officer she feels better knowing the maid is still there. He adds that she can call back if anything changes, and introduces himself as Officer Burroughs. After hanging up, Jill decides to check on Rosa, but can't find her. When she tries calling her phone she finds it in her purse.
Jill returns to the living room, and the phone rings again. When she answers, the same male voice asks, "Have you checked the children?" Worried, Jill goes upstairs and quietly opens the children's bedroom door. They are still asleep in bed and she heads back down—only for the phone to ring once more. This time, the voice says, "How were the children?" Horrified, Jill realizes the man can see her. She rushes to close the curtains and calls Officer Burroughs back, frantically explaining. He tries to calm her, insisting it's likely a prank, but Jill insists the caller "must want something." He tells her he can trace the call if she keeps him on the line for at least one minute. Jill agrees and tries to steady her breathing, preparing for the next call.
Following Officer Burroughs' instructions, Jill sets the timer on her watch. The phone rings again; she tries talking to him longer by asking him questions, but the man hangs up before the minute is up. Glancing out the window toward the guest house, Jill remembers that the Mandrakises' eldest son, Todd, sometimes comes home from college unannounced. She calls and leaves a voicemail that she has called the police and if he's home to come wait with her.
As she hangs up, a shadow passes by the window. Thinking it might be Todd, Jill quickly arms herself; and disarms the alarm, goes outside, then and rearms it before running across the path to the guest house. It's empty, with no sign that Todd has been home. The phone rings again; Jill answers and demands to know how the man got the number, but he hangs up. Checking her timer, she realizes he'd stayed on the line long enough for a trace—but he'd called the wrong phone line. Jill looks back toward the main house and sees a light switch on in Rosa's room. Believing it's her, she races back, disarms the alarm, and calls out for Rosa but gets no response. The phone rings again and while walking toward Rosa's room, she pleads for an answer, asking why he keeps calling. The man's calm voice tells her that what he wants is her blood on him.
Sickened, Jill hangs up and enters the bathroom and finds the shower running but no one there. As she shuts off the water, the phone rings once more. Expecting the stranger she angrily tells him to stop calling but instead hears Officer Burroughs, who urgently warns her that the calls are coming from inside the house and she needs to get out. Jill freezes in horror as the lights go out. A faint cellphone rings nearby, and when she turns toward the sound, she sees Tiffany's dead body on the floor. Screaming, she bolts from the room and into the darkness.
Moving through the darkened house, Jill goes to the children's room to collect them and escape. She pulls back the blankets on their beds, but they're gone. She checks the closet and finds them hiding inside a toy chest. She starts to lead them out, but the bedroom door is blocked by a dresser—and when she looks up, she finally sees the intruder crouched in the rafters above. Jill rushes the children back into the closet, securing the door with a cord as the man tries to break in. She tells the terrified kids to crawl through the small window that opens into the atrium, following close behind.
They reach the atrium, but the exit is locked. Jill watches the motion-sensitive lights flicker toward them, indicating movement. Dread turns to relief when it's only the cat—just before the intruder suddenly slams on the glass panels behind them. Jill quickly activates the sprinkler system, cloaking them in falling water. She hides the children, then lowers herself into the pond, ducking beneath the bridge. Underwater, upon suddenly discovering Rosa's lifeless body, she gasps and resurfaces in shock. As the man moves near the children, Jill signals for them to run, and they flee. But when the sprinklers shut off, the sound of their footsteps alerts him.
The kids escape as Jill hides under the bridge, but the man finds her and attempts to grab her. She kicks the bridge, knocking him into the water, and runs. He grabs her by the hair, but she forces the door closed, tearing free as she locks him inside. In the main room, Jill finds the children hiding near the fireplace—but spotting one of the atrium's birds on the stairs, she realizes the man has escaped. As she and the children run to the door the intruder appears and pulls her back as the children flee. A violent struggle ensues; Jill is nearly strangled before she smashes a bottle of alcohol against the fireplace and ignites it; the burst of flame throws him off. She drives a fire poker through his hand and bolts as he screams in agony.
Bursting through the front door, Jill collides with Officer Burroughs. In shock, she screams and fights him, believing him to be the killer, until he calms her and assures her she's safe. Outside the Mandrakis home, the children are reunited with their parents as Jill, still shaken, speaks with Officer Burroughs. When she asks where the intruder is, he tells her the man has been tranquilized and restrained, but Jill insists it isn't enough. As the police car drives past, she catches a glimpse of the killer's face glaring at her, leaving her horrified.
Later, in the hospital, Jill wakes to the sound of a balloon popping. She tries to page a nurse but gets no answer. She steps into the hallway and finds it empty and silent. When she returns to her room, the hospital phone begins ringing. She stares at it but doesn't answer and then looks toward her reflection in the mirror. As she closes the door, the stranger appears behind her and attacks. Jill wakes up screaming, thrashing in terror as her father and nurses try to hold her down and calm her; she cries that he's in the house, consumed by the trauma of her ordeal.
