On a rainy night, a vengeful psychopath, Tom Cooper, is parked outside of his ex-wife's house. He removes his wedding ring and then lights a match, staring blankly at it, and a bottle of hydrocodone. Tom grabs an axe and a tank of gasoline and breaks into the house and kills her and her boyfriend, and sets the house on fire. It explodes as he drives away, expressionless.

The following morning, Rachel Flynn, a newly divorced single mother, drives her son Kyle to school during rush hour traffic. As she is running late to work, one of her clients fires her over the phone. She then honks at Tom's pickup truck, which is blocking traffic at a green light. He soon catches up to Rachel and tries to apologize, then asks for an apology in return, saying he wants to "hit reset", and that "people should apologize for anything they do wrong". The exchange escalates when Rachel tells him that she has nothing to apologize for. In return, Tom promises to teach her "what it means to have a bad day". At the end of the road, Tom drives in front of Rachel and stops. He then drives away. Unsettled, she calls her friend Andy, who is also her divorce lawyer, and arranges to meet him at a diner.

Tom somehow tracks Rachel to a gas station and switches his phone with hers while she is inside paying for her gas. She sees him outside and gets help from a customer who accompanies her back to her car. The customer gets Tom's license plate number but is killed not when Tom rams him with his truck, but after another car hits him. A more intense chase ensues, during which Tom shows Rachel that he has her phone. He uses her daily planner to locate Andy at the diner. Before she can warn Andy, Tom beats and stabs him to death in front of everyone, and is filmed by a customer. Tom tells Rachel over the phone that she must choose another one of her contacts to die next. When he suggests her ex-husband, her mother, or her son, she names the client who fired her, then calls the police, who respond to the client's home while Rachel is racing to her son's school.

However, Tom has already arrived at Rachel's home, where her brother Fred and his fiancée Mary are living. Tom confronts him, holding Mary hostage, before repeatedly pushing her into Fred's knife, killing her. He then ties Fred to a chair and tells Rachel over the phone that she has three minutes to go into the school, get Kyle, and drive away, or he will kill Fred, as he wants him to read a letter about how Rachel is a liar. Rachel desperately demands that the principal release Kyle. She gets him and drives away. When a police officer arrives at Rachel’s house, Tom sets Fred on fire. The cop shoots Tom in the shoulder and tries to put the fire out, but Tom escapes and catches up to Rachel and Kyle on a highway. They use a GPS app on her tablet to find out that Tom is in a minivan (it is indicated he killed someone for it), and is directly in front of them. When they attempt to alert a nearby police officer, Tom rams his car, causing a massive multi-vehicle car accident.