The movie tells the story of Sharon Gill, a modern-day Canadian Sikh graduate student who travels to India as part of her thesis on minorities at war. Revelations about the past soon leave the student curious for more information, and Gill's awareness changes after maneuvering into prison to interview Kartaar Singh. Singh is a hockey player whose life experiences lead him to fight against corruption in the police and government systems. The film's central story is based around Kartaar Singh's journey which is filled with courage, perseverance, and faith.

Stray, poor, idle, addicted common people can be found easily around us. The politicians and system treat them as puppets. Such people are not considered as human beings. Most of their questions can't be solved but they themselves are a big question for society. These "unimportant" people either get trapped or become revolutionary for the society. Revolutionary people serve as the flag of humanity, the colors of which cause irritation in other's eyes. Posti is the story of the irritation caused by these revolutionary colors. Why it happened in the past, happening now and how long will it continue?