In the days of the British Raj in 1919, Captain Sam Wyndham, a former Scotland Yard detective newly arrived in Calcutta, is confronted with the murder of a British official who was found with a note in his mouth warning the British to leave India.

Calcutta, 1919. Captain Sam Wyndham, former Scotland Yard detective, has been recruited to head up a new post in the police force. The body of a senior official has been found in a filthy sewer, and a note left in his mouth warns the British to quit India, or else. Wyndham is teamed with arrogant Inspector Digby and Sergeant Banerjee, one of the few Indians to be recruited into the new CID. The case takes them from the opulent mansions of wealthy British traders to the seedy opium dens of the city-- and puts them under pressure to solve the case before it erupts into increased violence on the streets.