Roshan (Abhishek Bachchan) takes his ailing grandmother Annapurna (Waheeda Rehman) back to their old family home in Old Delhi, where he is immediately immersed in the energetic and chaotic lives of their neighbors. Among them are the cultured Ali Baig (Rishi Kapoor), the feuding siblings Madangopal (Om Puri) and Jaigopal ([Pavan Malhotra]), their wives, and many more vibrant personalities who make up this close-knit community.
Facing a terminal brain tumor, Annapurna has a limited time left, and it is her wish to revisit her roots. Leaving behind his life in the US, Roshan steps in after his father declines to accompany her. The neighborhood is a tapestry of Hindu and Muslim life, vibrant with traditions and beliefs, including a revered tree temple held dear by the locals.
Roshan soon gets acquainted with the neighbors, including Rajjo Bhabhi ([Sheeba Chaddha]) and Vimla ([Supriya Pathak]), and finds himself enchanted by the local culture. He accompanies Annapurna to the lively Ramleelas, indulges in treats from Mamdu's ([Deepak Dobriyal]) sweet stall, and becomes a familiar face to the local children. However, when Annapurna's health takes a downward turn one day, Roshan is thrown into the midst of a surprising spectacle as locals gather around a cow in labor, prompting him to confront the deeply embedded social customs that influence their lives.
He discovers that Madangopal's unmarried sister Rama ([Aditi Rao Hydari]) and Jaigopal's ailing business epitomize the challenges underpinning their community. The plot thickens with love affairs and secret ambitions: Shashi ([Geeta Bisht]), the wife of the local money lender, is involved with Suresh ([Cyrus Sahukar]), who also has designs on Madangopal's daughter Bittu ([Sonam Kapoor]), a talented singer with aspirations for the reality show Indian Idol.
As Roshan offers support to Bittu, he finds himself attracted to her, and she begins to return his feelings, despite the looming specter of his imminent departure. When familial pressures threaten Bittu's dreams, Roshan intervenes, stirring the ire of her father Madangopal. Yet, Bittu determines to pursue her passions, seeking to elope with Suresh, unbeknownst to his deceptive double life.
In the midst of this, the community is shaken by fears of a local miscreant known as the "Kala Bandar" who wreaks havoc, causing fatalities. Roshan's situation intensifies when he becomes a suspect in the chaos surrounding the Kala Bandar sightings, which further complicates his budding relationship with Bittu.
As tensions rise between Hindus and Muslims, even igniting a local riot that threatens to upend their peaceful coexistence, Roshan finds himself at a crossroads, torn between love and loyalty to his grandmother, who wishes to die in her ancestral home. His cultural heritage becomes a fulcrum for conflict, especially as the mob's anger escalates, leading to tragic consequences for many.
In a climactic confrontation, disguising himself as the Kala Bandar in a final attempt to disrupt Bittu's forced marriage, Roshan publicly reveals his feelings for her, only to face backlash from the community. In the chaos, Gobar makes a profound statement that the true Kala Bandar lies within themselves, representing their internal struggles rather than an external enemy.
As Roshan's life hangs by a thread, he experiences a spiritual awakening, meeting the spirit of his deceased grandfather ([Amitabh Bachchan]) who teaches him about the true nature of hatred and division. Resurrected by the compassion of his community, Roshan ultimately resolves to embrace his new home in Delhi 6, where unity in diversity becomes his guiding principle. He realizes that the true Kala Bandar is not a figure to be hunted but a challenge to be faced from within each soul, declaring that "Who can ever wish to leave Delhi's lanes?" in a heartfelt affirmation of his commitment to this rich tapestry of life.
