Julia is a college student who takes a babysitting job for Dr. Mandrakis and his wife. The Mandrakis' are heading out for the evening, and Julia is left alone in their big, isolated house with their two young children. At first, everything seems fine, the kids are sound asleep, and Julia settles in, with the Mandrakis' instructions to just call if she needs anything.
However, things take a turn for the worse when Julia receives a series of eerie phone calls. Each time she answers, there's no one on the other end of the line. Julia begins to feel increasingly anxious as the calls continue and becomes convinced that someone is watching her. Then, the phone rings one more time, and this time it's the stranger requesting help.
Julia agrees but finds that the phone line is dead, and when she goes outside to look for the stranger, he's nowhere to be found. Things start to get even more strange when Julia returns to find that the children are missing. She contacts the police and begins to work with them to try and find the children, but it seems like they've vanished without a trace.
Days go by with no sign of the children until the stranger calls Julia again. He's using a voice modulator to disguise his voice and toys with Julia, telling her that he's taken the children and warning her not to involve the police. Julia tries to stay calm and follow the stranger's instructions to get the children back.
The stranger sends Julia on a series of tasks and is clearly enjoying manipulating her. Through a series of clues and puzzles, Julia pieces together a plan to rescue the children. She manages to outsmart the stranger and rescues the kids, but not before some incredibly tense moments and a climactic confrontation with the stranger.
The film ends with Julia, the children, and the police safe, but Julia clearly traumatized by her experience. When a Stranger Calls Back is a tense, unsettling thriller that will keep viewers on the edge of their seats.