The film opens with a senior Indian Army Officer Lt. Gen. Bakshi addressing a group of soldiers on board the INS Viraat. The story flashes to the events preceding the 1971 India-Pakistan war. Sehmat is a college student from Kashmir. Her father and grandfather were Indian freedom fighters. She learns of her father's impending death from cancer and his last wish for her to continue the family tradition of being in service to the country. She is soon to get married to an officer, the son of an Army Brigadier in the Pakistani Army, who is promoted to Major General. A few days before the marriage ceremony, Sehmat is hastily trained by members of RAW, India's external intelligence agency, in various skills required to be a spy.
While being trained at RAW, Sehmat learns the contact numbers to be used in future while contacting anyone from Pakistan. She practices converting text information to morse code, which she had to use while transmitting information to India.
After the marriage and migration to Pakistan, Sehmat quickly settles into her married life, adjusts to a new country, and establishes the trust and confidence of her in-laws. She also falls in love with her husband Iqbal, though she shortly establishes communication channels with her handlers back in India and starts relaying information. Eventually, she spots information related to the planning of some sort of offensive against India and, at great risk, is able to gather the necessary details and pass them on to her handlers.
It turns out that this information points to a possible attack on the Indian aircraft carrier INS Vikrant, then deployed in the Bay of Bengal. The storyline suggests that this information provided one of the initial warnings to the threat which could have some bearing on the sinking of the submarine PNS Ghazi off Visakhapatnam's coast.
Toward the end, Sehmat is discovered as a spy but manages to escape with the help of her handlers, at the cost of Iqbal's death. She ultimately returns to India, after which it is discovered that she is pregnant with Iqbal's child. She says that she will raise the boy alone; the boy, Samar Syed, later joins the Indian Army.
In the concluding scene, an aged Sehmat sits alone at the window of a small house.