A prank phone call leads to a night of pure terror. With her parents away on an overnight trip, Libby amuses herself by joining her friend Kit, in calling up strangers and whispering, 'I saw what you did.' When Libby dials Steve Marak, a psychopath who just murdered his wife, his jealous neighbor listens in on the call, a conversation that ends with the panicky killer determined to silence them all.