The film begins on a Turkish railway, 400 miles from Istanbul, where Robert McCall enters the bar cart and shares a drink with a Turkish man named Adam Karst, who has abducted his daughter out of spite for her American mother. Upon realizing McCall's intentions, the man instructs his henchmen to deal with him. McCall swiftly sizes up his adversaries, sets a timer, and with a series of swift and brutal moves, subdues them all. He ominously approaches the father, offering him a chance to choose an easier path.
Meanwhile, in Boston, a distraught Grace (Tamara Hickey) visits her lawyer's office, where her daughter awaits her. The emotional reunion sees her embracing her young girl with tears in her eyes.
As McCall adapts to life as a Lyft driver, he often rides with an elderly man named Sam Rubinstein, who is desperately searching for his long-lost sister. One fateful night, McCall takes a young woman named Amy home after she was drugged and assaulted. He promptly takes her to the hospital, before confronting her attackers at their apartment. When the leader attempts to strike him, McCall asserts dominance, injuring all of them before leaving.
In a parallel storyline in Belgium, a man named Mr. Calbert (Antoine de Lartigue) comes home to find his wife, played by Abigail Marlowe, brutally beaten. A trio of thugs—Resnik, Ari, and Kovac—enters and executes her before compelling Mr. Calbert to take a seat, where Resnik forces a gun into his mouth and pulls the trigger.
Later, while at a bookstore, McCall crosses paths with Grace, though she remains unaware of his role as her daughter's savior. Back at his home, he enjoys a friendly dinner with Susan Plummer, only for them to momentarily forget and reminisce about his late wife Vivian's birthday.
Noticing vandalism in the garden of a neighbor, Fatima, McCall begins cleaning it up and gets help from Miles Whittaker, a young aspiring artist who frequently interacts with him. Meanwhile, Susan and her colleague, Dave York, are investigating the Calberts' murder and travel to Belgium for further inquiry.
Susan finds herself attacked in her hotel room by two thugs but bravely defends herself until she is tragically cornered. Receiving a call from Brian, Susan's husband, McCall learns of her murder and travels to Washington, D.C., for her funeral. As he delves into her death, reviewing hotel footage and photos, he uncovers a web of connections to the Calbert case.
As the investigation unfolds, darkness creeps back into McCall's life, culminating in confrontation after confrontation with the thugs who have disrupted his world. McCall feels compelled to protect Miles after he learns he was involved in a revenge plot against the murderers of his brother. Knowing he must intervene, he persuades Miles to hide as tensions rise.
A hurricane forces McCall to return to his hometown, where the villains seek him out. A fierce showdown occurs in which McCall uses his skills to eliminate his enemies in cinematic fashion, utilizing a harpoon and explosives to exact his revenge. Ultimately, he confronts Dave atop a tower, leading to a climactic battle that leaves Dave dead and reveals the truth behind the betrayal.
After saving Miles, who has been injured, McCall treats his wounds and reflects on his past, culminating with a poignant mural painted by Miles that captures the essence of McCall's influence.
In a touching conclusion, Fatima finds that her garden has been restored, and Miles, filled with inspiration, begins bridging friendships through art. Sam Rubinstein is also seen joyfully reuniting with his sister, further highlighting the film's themes of connection and healing. As the story closes, McCall stands gazing out at the ocean, a visual metaphor of both his past and the potential for a better future.
