In "After Twenty Years," the theme of change is central to the narrative. The story explores how time alters people and circumstances, often leading to unexpected outcomes. Two friends, Bob and Jimmy, who were once inseparable, find themselves on divergent paths after two decades. Their reunion, set against the backdrop of a dark, lonely New York street, reveals the profound changes that have occurred in their lives and characters.

Bob and Jimmy were close friends in their youth, almost like brothers. They shared a deep bond and made a pact to meet again after twenty years. However, their lives took different directions. Bob became a criminal, constantly on the run, while Jimmy remained a law-abiding citizen, eventually becoming a policeman. This change in their characters is evident when Bob, once full of confidence, becomes a shaken man upon realizing his friend has turned him in. The story suggests that the passage of time can transform a good man into a bad one, as seen in Bob's descent into crime.

The setting of a dark, windy night in New York City enhances the theme of change. The deserted street symbolizes isolation and the passage of time. The once vibrant 'Big Joe' Brady's restaurant, where the friends made their pact, is now a closed hardware store, reflecting the changes in both the city and their lives. The darkness also plays a crucial role in the story's twist, as it conceals identities and intentions until the climactic revelation under the bright lights of a drugstore.

The appointment between Bob and Jimmy was meant to celebrate their enduring friendship. However, it becomes a poignant reminder of how time can alter relationships. Despite their intentions, the changes in their lives and characters lead to an ironic and bittersweet reunion. Jimmy's duty as a policeman outweighs his old friendship, illustrating how time and circumstances can redefine personal bonds.

O. Henry masterfully uses the setting and character development to illustrate the inevitability of change. The story's surprise ending, a hallmark of O. Henry's style, underscores the irony of time's impact on friendship and identity. Through "After Twenty Years," the author poignantly captures the essence of change, revealing its power to reshape lives and relationships in unexpected ways.