In November, 1971, India and Pakistan are on the verge of declaring war on each other, as a result of the refugee crisis precipitated by the Pakistan Army under Operation Searchlight. The Indian Navy and RAW decipher a secret code sent from Pakistan to East Pakistan (modern-day Bangladesh), relating to a possible attack on an Indian naval vessel. The Navy deduces that the target may be INS Vikrant, India's sole aircraft carrier. The top command dispatches the submarine S21, under the command of Cmdr. Ranvijay Singh for recce missions. To temper Singh's belligerent tendencies, the Indian Navy Admiral assigns Lt. Cmdr Arjun Varma to S21, with orders to ensure that Singh does not confront any Pakistani warships and trigger a war. The Navy's top command soon learns that the Pakistan Navy has dispatched PNS Ghazi, under Cmdr. Razak Khan, to the Bay of Bengal to confront Vikrant. Meanwhile, to divert the attention of the enemy, Ghazi torpedoes an Indian merchant ship. The attack is picked by S21, which races to the scene. Noticing survivors in the wreckage, Arjun jumps into the sea and manages to rescue a girl child and a woman, both of whom are Bengali refugees. During surveillance, S21 acquires a sonar signal of Ghazi. Singh believes that they should track down Ghazi and attack, while Arjun obstructs, reiterating his instructions. Singh then orders the ship's EXO, Lt. Commander Santosh Devraj to conduct a drill and target Ghazi while doing so, despite Arjun's protests. They fire a torpedo, which misses Ghazi narrowly, thus alerting of their presence. Aware of the enemy's presence, Razak orders his crew to head towards Visakhapatnam Port at full speed and to set up mines en-route, planning to destroy S21. S21 pursues Ghazi's path and realizes at the last moment that it is headed into mines. Singh and Arjun frantically attempt to change its course but a mine explodes in the stern and damages most of the sub's batteries, propellers and circuits. The sub blows open many leaks, with the forward torpedo compartment flooded, crippling the sub and barring it from firing its forward torpedoes. Singh dies while trying to save Arjun, as S21 sinks to the sea-bed.