Set during the Prohibition era in the United States, "91 Days" unfolds in the fictional town of Lawless, Illinois, a hotbed of organized crime and bootlegging. The story centers on Angelo Lagusa, a young man whose family is brutally murdered by the Vanetti mafia family when he is a child. After witnessing the massacre, Angelo flees and assumes a new identity, living in obscurity for years. However, a mysterious letter reignites his desire for vengeance, prompting him to return to Lawless under the alias Avilio Bruno.

Avilio infiltrates the Vanetti family by befriending Nero Vanetti, the son of the don and one of the men responsible for his family’s death. Over the course of 91 days, Avilio meticulously manipulates events to sow discord within the Vanetti organization, exploiting rivalries and betrayals to bring about its downfall. His calculated actions lead to a series of violent confrontations, shifting alliances, and moral dilemmas, as he navigates the treacherous world of mafia politics.

Nero Vanetti, initially portrayed as a carefree and impulsive heir to the family, becomes a central figure in Avilio’s plan. Despite their growing camaraderie, Avilio’s true intentions remain hidden, creating a tense dynamic between the two. Other key characters include Corteo, Avilio’s childhood friend who aids him in his quest, and Fango, a rival gang leader whose chaotic presence further destabilizes the power balance in Lawless.

The narrative explores themes of revenge, loyalty, and the cyclical nature of violence, as Avilio’s actions have far-reaching consequences for both himself and those around him. The story’s pacing is deliberate, with each episode marking a day in Avilio’s 91-day journey, culminating in a climactic resolution that challenges the notions of justice and retribution. The series maintains a grounded tone, emphasizing the moral ambiguity of its characters and the harsh realities of their world.