An unknown woman is locked in a fierce struggle with a man inside a vehicle, ultimately ending the confrontation with a decisive shot to his head. Five years prior, the same woman, Riley North, is depicted as a banker in Los Angeles, fighting to make a living. Her husband, Chris North, runs a struggling mechanic shop. They celebrate their ten-year-old daughter, Carly North, with a pizza outing and a birthday trip to a carnival, in an attempt to make up for the absence of friends at her party. At the carnival, Carly picks peppermint ice cream, which serves as a significant detail linked to the film's title. Tragically, as they return to their car, gunfire erupts from the men of Diego Garcia, a notorious drug lord, resulting in the brutal murder of both Chris and Carly. Though wounded, Riley miraculously survives the attack.
Despite her injuries, Riley is able to positively identify the assailants. However, the detectives on the case are reluctant to move forward, hindered by the immense power of Garcia’s drug cartel. Detective Moises Beltran informs his colleague Detective Stan Carmichael that a previous detective who dared to challenge Garcia met a gruesome fate, prompting fear of reprisal. As Riley anticipates the preliminary hearing, the lawyer of the shooters attempts to bribe her. She refuses, but he preys on her vulnerability by highlighting her anti-psychotic medication, aiming to depict her as an unreliable witness. Judge Stevens, who has his own dealings with the cartel, dismisses the case citing lack of evidence, leaving Riley infuriated as her family's killers walk free. After a desperate attempt to assault the men responsible, she finds herself tasered and confined to a mental institution, where she successfully escapes and disappears from the public eye.
Fast forward five years: Detectives Beltran and Carmichael are called to the carnival, where they discover the three shooters executed and displayed on the Ferris wheel—an act orchestrated by Riley. Her actions spark the interest of FBI Agent Lisa Inman, who reveals that before her disappearance, Riley had robbed the banking institution she once worked for and is now back in action after raiding a gun store for weapons. Riley's vengeance escalates; she takes out Judge Stevens using an explosion, having already eliminated the defense and prosecution lawyers from her family's case. Inman, Beltran, and Carmichael find themselves entangled in a media storm as public opinion fractures between viewing Riley as a vigilant hero or a criminal.
Determined, Riley infiltrates one of Diego's money laundering operations, dispatching many of his men but saving one for interrogation. Upon realizing Riley is behind his recent shipments' disappearances, Diego vows to take her down. Meanwhile, Inman discovers Riley has been hiding out on Skid Row, where the homeless community views her as a guardian angel amidst rising crime. A confrontation with Diego ensues when Riley attempts to execute him, but she hesitates when faced with his young daughter, reminiscent of her lost child, resulting in her being injured while Diego escapes.
Inman, suspecting Riley's whereabouts, calls Carmichael to Skid Row, unaware that he is secretly allied with Diego. Betrayal unfolds as Carmichael shoots Inman before notifying Diego of Riley’s probable location. Riley returns to the chaotic streets of Skid Row, which are now overrun by Diego's henchmen. Despite the overwhelming odds, she fights through them and discovers Inman's lifeless body. Seizing Inman's phone, Riley reaches out to the media, exposing her location and inviting both the press and LAPD to the scene. In a tense showdown, she faces Diego, delaying him just enough for police to arrive. In the fray, Diego ends up shooting Carmichael, but before he can escape, Riley confronts him, landing a final blow.
Amidst the chaos, Riley is found in critical condition at her family's grave by Beltran, who defies her wish to perish and rushes her to the hospital. Later, he pays her a visit, discussing the support for her actions and discreetly providing her with the key to her handcuffs, allowing her to plot yet another escape.
