Paul Kersey, a trauma surgeon in Chicago, lives with his wife Lucy and college-bound daughter Jordan. A restaurant valet, Miguel, photographs the home address from their car's navigation software after overhearing that they will be out one night. Their plans change when Paul is called to work. Jordan and Lucy return home and are confronted by three masked burglars who kill Lucy and leave Jordan in a coma.
Police detective Kevin Raines is tasked with the case, but Paul soon becomes frustrated with the lack of progress. One night, Paul comes across two thugs harassing a woman and tries to intervene but gets beat up. When an injured gang member is brought to the hospital and his Glock 17 falls off the gurney, Paul takes it, using online videos to learn how to use it. Paul uses it to stop a carjacking, a video of which goes viral; he is dubbed Chicago's "Grim Reaper". His reputation as the Grim Reaper soars after he kills a drug dealer known as "The Ice Cream Man". When Miguel is hospitalized, Paul finds that Miguel is wearing his stolen watch. After Miguel dies, Paul takes his phone and finds information leading him to a liquor store that fences stolen goods.