Teenagers Libby Mannering and Kit Austin are having a night of innocent fun while babysitting Libby's younger sister, Tess. Bored and looking for excitement, they start making prank phone calls to random numbers in the phone book. They stumble upon a call with the simple but ominous message: "I saw what you did, and I know who you are." Unbeknownst to them, they call Steve Marak, a man who has just committed a murder. Startled and intrigued by the cryptic message, Steve becomes desperate to find out who the callers are to silence them and cover his tracks.

As Steve attempts to track down the girls, Libby and Kit find themselves plunged into a series of increasingly dangerous situations. Their harmless game turns into a terrifying ordeal as they realize they are being pursued by a relentless and calculating killer. Tensions rise as the reality of their situation becomes clearer, forcing the girls to confront the consequences of their actions while desperately trying to stay one step ahead of the murderer. What started as a simple prank quickly spirals into a nightmare as they navigate a web of fear and danger.