Saint Petersburg's police major Igor Grom is on a thrilling pursuit of three bank robbers clad in striking red sports suits, a direct homage to the villainous hockey players from the beloved Soviet cartoon Puck! Puck! In a moment of intense chaos, he apprehends the thieves but not without causing considerable property damage, leading to his reprimand by his superior, Colonel General Fedor Prokopenko.
Shortly after, a sensational trial begins for "rich boy" Kirill Grechkin, accused of the tragic hit-and-run that claimed the life of a young girl. Despite overwhelming evidence against him, Grechkin walks free, much to the dismay of the victim's brother, Lyosha Makarov. Fueled by tragedy and corruption, Sergey Razumovskiy, the affluent IT millionaire and founder of the social networking site Vmeste, alongside his childhood friend Oleg Volkov, embarks on a quest for justice. As public outrage simmers, an enigmatic figure known as the Plague Doctor emerges online, a masked vigilante with plans to "cleanse" St. Petersburg of its scoundrels. The Doctor’s deadly mission escalates when he sets Grechkin ablaze in a brutal act of retribution.
In the wake of this murder, Oleg confesses to Sergey that he is indeed the Plague Doctor, causing a rift between the friends as the stakes rise. Major Grom is keenly aware of the doctor’s charisma and the following he garners, as he investigates the murder but faces resistance from Evgeniy Strelkov, an FSB officer dispatched from Moscow to capture the elusive Doctor. Stripped of his official investigation, Grom, alongside his eager rookie partner Dima Dubin, is dispatched on a seemingly mundane case involving the theft of twelve refrigerators. Yet, their minds remain focused on the Doctor, fueled by intel from Grom's informant.
As the tension mounts, Grom finds himself rescuing a woman from thugs, only to discover she is Yulia Pchyolkina, a journalist who staged the attack to gain insights into the Plague Doctor for her blog. Meanwhile, the Doctor grows emboldened, targeting not just the corrupt but anyone deemed a villain, including a banker engaging in scams and the owner of a toxic landfill, indiscriminately exterminating their families as well.
With the casino opening looming, Dubin hypothesizes that the Doctor may strike again, and Grom infiltrates the high-profile event. However, instead of facing the infamous villain, chaos erupts when the Doctor's followers descend upon the wealthy guests in an attempted robbery. Razumovskiy steps in to mediate the violence, but a clash ensues, resulting in the capture of the thieves. This action, however, culminates in Grom's forced resignation from the police.
Though dismissed, Grom's determination remains unwavering. Through investigation, he uncovers Razumovskiy's past, including his time in the orphanage where he would draw birds reminiscent of the Plague Doctor's mask. The tension crescendos as Grom confronts Razumovskiy, at which point shocking revelations unfold: Razumovskiy acknowledges his split existence, revealing Oleg’s death was a cover for his own hidden identity. In a fateful turn, Razumovskiy dons the Doctor's attire, setting Grom up for capture as he prepares for his next act of vengeance.
As riots erupt in the city, Razumovskiy divulges to Grom his grandiose vision for a "new Petersburg," intending to eliminate both his followers and those he holds in disdain. In a climactic showdown, Grom battles Razumovskiy, who is ultimately subdued with the aid of Dubin and Pchyolkina. With evidence of Razumovskiy's murderous intent recorded, Grom implores that violence is not the solution to societal issues.
The chaos eventually wanes with Grom’s informants managing the rioters, while Lyosha grapples with his conscience, opting to spare the life of the corrupt judge instead of succumbing to revenge. Strelkov takes all the glory for the capture of the Plague Doctor, leaving Grom to cherish his newfound camaraderie with Dubin and Pchyolkina.
In a gripping mid-credits scene, Razumovskiy, now confined to a mental institution, engages in a conversation with his dark alter ego, hinting at impending liberation. The post-credits reveal Oleg Volkov, alive and well, observing his friend's demise from a militant camp in Syria, suggesting that the shadows of their past are far from extinguished.
