A Pakistani submarine, Ghazi plans to secretly attack Vizag port. For doing so, it has to get past Indian submarine S21.
In 1971, amid rising tensions between India and Pakistan over the liberation war of East Pakistan, Pakistan navy, in a top secret mission, plans an attack on Indian majestic-class aircraft carrier INS Vikrant, to gain control over the waters of Bay of Bengal. They send their best-in-class submarine PNS Ghazi, but are intercepted by an Indian submarine INS Karanj (S21). An underwater attack ensues.