As Dr. Park (/actor/sandra-oh), a dedicated psychiatrist, explores the troubled mind of vigilante Arthur Poppington, more commonly known as "Defendor," the story unfolds with flashbacks that reveal a significant moment between Arthur and a corrupt detective named Dooney. Arthur is convinced that Dooney is colluding with his formidable enemy, Captain Industry, a mysterious figure who has haunted him since childhood. This torment began when Arthur's mother left him, leading to her tragic overdose, a fate Arthur attributes to the schemes of Captain Industry.
After an incident of assault against Dooney, Arthur's life takes a drastic turn as he confronts the daunting realities of the justice system. Captain Fairbanks, a veteran police officer who views Arthur as more of an annoyance than a threat, forms an unusual connection with him, having discovered their shared heritage—both grandfathers were soldiers in World War I.
Returning to his isolated existence at a government construction site, Arthur’s peace is shattered yet again when he encounters the violent henchmen of Dooney. During this turmoil, he meets Angel, a street-smart and resilient prostitute who had been sharing crack with Dooney when Arthur found himself in trouble. After suffering a brutal beating, Arthur is rescued by Angel, who discloses the true identity of Captain Industry as Radovan Kristic. Armed with this critical insight, Arthur enlists Angel's aid in bringing down Kristic.
Concerned for Arthur's safety, his close friend Paul notices the rising tension as Arthur takes Angel in, offering her shelter at the construction depot. Despite Paul's attempts to steer Arthur towards a safer path, the determined vigilante dismisses his friend’s help.
That same night, Arthur, driven by a sense of purpose, decides to investigate Kristic and Dooney further. However, his plan takes a dark turn as he finds himself ambushed, brutally assaulted, and shot. Angel discovers him in this vulnerable state and contacts Paul, who ensures Arthur receives the medical attention he needs.
While Arthur battles for his life post-surgery, Paul shares his worries with Angel, advising her to keep her distance from the spirited vigilante. But Angel disregards this advice, visiting Arthur and revealing her own tragic past of abuse. Little does she know, Arthur, in a semi-conscious state, hears her heartfelt revelations.
Revitalized by a sense of purpose, Arthur embarks on a mission to confront Angel’s abuser, aligning his goals with hers. As Dr. Park reflects on the complexities of Arthur's situation, she passionately defends him in court, emphasizing his cognitive challenges due to fetal alcohol syndrome. The judge, moved by her argument, decides to show leniency, provided Arthur abandons his alter ego, Defendor. Meanwhile, Paul opts to allow a journalist to profile Arthur, capturing the public's fascination with the elusive vigilante.
While Angel struggles with her addiction, she finds herself back in Dooney’s orbit, who grows increasingly wary of what Arthur may know about their criminal activities. In a desperate effort to suppress Arthur’s voice, Dooney takes Angel hostage, threatening to expose Arthur’s secrets. Misunderstanding this threat as a directive to cease speaking, Arthur decides to retreat into silence, presenting an air of mystery to the world.
Angel ultimately escapes from Dooney's grasp, and when she reconnects with Arthur, she is shocked to learn of his plan to confront Kristic. In a moment of raw emotion, Angel confesses her long-standing desire for revenge against Kristic, blaming him for her mother's demise.
A poignant flashback reveals that Arthur’s grandfather shielded him from the truth of his mother’s death, attributing blame to “the captains of industry,” thus cultivating Arthur’s lifelong obsession. This insight helps Angel understand the extent of Arthur's delusion and the role she has played in it.
As Arthur prepares for a confrontation with Dooney and Kristic, he proves his mettle by taking down multiple henchmen before being grievously shot by Kristic. In his final moments, Arthur accuses Kristic of murdering his mother, with Kristic’s reaction hinting at his possible involvement in her tragic end.
Holding Arthur in her arms, Angel vows to transform her life, determined to quit drugs and seek stable employment. In previous discussions, Arthur had recognized her writing talent, comparing it to that of Lois Lane. Inspired, Angel sits at her typewriter, prepared to immortalize Arthur’s legacy through her stories.
In the end, justice prevails as Dooney faces his consequences in prison and Kristic is extradited to Serbia. A heartfelt memorial service for Defendor is held, attended by Dr. Park and her daughter, beneath a vibrant mural celebrating his unconventional heroism. The film closes with Angel diligently working at her typewriter, weaving tales that honor the memory of Arthur Poppington, the beloved vigilante known as Defendor, forever etched in the fabric of history.
