The Forgotten Army is based on a chapter in Indian history, during the peak of India’s struggle for freedom from British colonial rule. It's about the soldiers who fought for the independence of their country, only to be forgotten.

An introverted old man (Old Sodhi) has come to visit his sister (Urmila) and her family, Subhash (Urmila’s son) and Amar (Subhash’s son) in Singapore. In an intervention by his older sister, Old Sodhi is catapulted into his past, the war of ’42, the past he was running away from. He narrates the controversial outcome of the Battle of Singapore and fate of the soldiers of the British Indian Army.

After surrendering, the fate of the soldiers depended on the mercy of the Japanese Army. The Indian soldiers transformed when Netaji gave the call to form the Indian National Army. Maya, an aspiring photographer, is driven to join this freedom army. Sodhi and Arshad realise that India is their motherland and that they have to fight to free her from British raj.

Maya and Rasammah enroll in the women’s regiment. Rajan and many other civilians also enroll in the INA. Rigorous training sessions begin for all the recruits. In 1996, Amar reaches Burma with Old Sodhi and they meet Rani (Amar’s Burmese friend). Circumstances lead to Old Sodhi shooting a Burmese soldier which paves way for a chain reaction of dangerous events.

Maya and Sodhi along with the INA soldiers do not give hope even after their trains are bombed by planes. Old Sodhi finds himself in Rangoon that leads to an array of past events - the struggles of the Indian soldiers and the rains that destroyed everything. Under such a situation, the Japanese were ordered to retreat.

There is an attack on the advanced base camp by the British Air Force but that doesn't deter the soldiers from marching ahead to stop the British from advancing. Sodhi orders all the wounded to retreat along with the Rani Jhansi Regiment. Old Sodhi injures his leg in the jungle and is unable to walk any further. Amar leaves with Rani to get help but are stopped on the tracks by a loud blast.