Manish was a Hindu nationalist during the Partition of India. Despite the fact his family and Umbreen's had lived and farmed together over the years, Manish never liked her. He misunderstood why his brothers fought in the Second World War, believing they shared his belief in only fighting for Hindus; when the war was over, Manish had "a brother that was dead and one [he] wish[ed] never came back."
Not wanting his brother to marry a Muslim woman, Manish tried subtly to dissuade the marriage from happening. However, when this didn't work, he grew desperate enough to kill Bhakti, a holy man, in an attempt to prevent the wedding.
Despite his efforts, the Thirteenth Doctor took Bhakti's place and wed Prem and Umbreen. Disgusted, he joined the other nationalists/robbers and lead them back to his farm, where he had Prem shot for not joining them. However, Prem had bought Umbreen, her mother and Team TARDIS time to escape his rage. (TV: Demons of the Punjab)