Plot
Set in 1945, during pre-independent India, Agent Devil follows the story of an officer working for the Indian Political Intelligence (IPI) who is sent undercover to investigate a murder in Rasapadu, a village under the Madras Presidency. The case revolves around the death of Vijaya, the daughter of a wealthy landlord. Vijaya's father is initially arrested as the primary suspect, with the motive being his shame over Vijaya's love affair with Bhuma, a domestic worker in their household. However, Agent Devil quickly begins to suspect that the landlord is innocent and that Vijaya's death is part of a larger conspiracy.
As the investigation progresses, Bhuma goes missing, and Rani, a cook in the household, is found dead. Bhuma is later discovered alive but is soon killed along with his father, Ramana, by an unknown assailant. Parallel to the investigation, Agent Devil develops feelings for Nyshadha, the landlord's niece, who is a London-educated barrister. Although initially hostile toward him, Nyshadha eventually reciprocates his feelings after he secures the landlord's acquittal in Vijaya's murder case.
Nyshadha, however, harbours secrets of her own. She and her companion, Samudra, are members of the Indian National Army (INA), led by Subhash Chandra Bose. Nyshadha reveals to Devil that a crucial code linking to the INA has been stolen, and they suspect it is related to Vijaya's murder. Devil learns that Bose is planning to return to India, with his landing location known only to Trivarna, Bose's close confidant whose identity is a mystery. Meanwhile, Clement, a British officer, is assassinated by Trivarna, who is revealed to be Manimekala, an Indian National Congress leader, as revenge for killing an INA agent.
Agent Devil's true mission in Rasapadu is gradually revealed: he was sent by the British to investigate Nyshadha after she decoded and transmitted a message about Bose's landing. The codes were leaked by Agent Trojan to General Kenneth Bracken, who then tasked Devil with tracking down Nyshadha. When the first part of the code, revealing the time and location of Bose's landing, is cracked, Bracken orders Nyshadha's execution, but Devil saves her. Devil convinces Bracken that Nyshadha needs to stay alive to deliver the final "All Clear" message to Trivarna, which is essential for Bose's safe landing.
Nyshadha is arrested but is later rescued by Devil, making her a fugitive. With a bounty on her head, she entrusts Devil with the task of contacting Trivarna. Devil sends a letter with the decoded message, which reaches Manimekala. Meanwhile, Shafi, a close ally of Trivarna, warns the INA that the code has been compromised, and they plan to change Bose's landing location.
Bracken eventually discovers that Manimekala is Trivarna. Devil brings her to Bracken, but in a surprising turn of events, he kills the soldiers tasked with executing her and reveals that he is the real Trivarna. Devil had joined the IPI on Bose's orders, not to investigate Nyshadha, but to identify Agent Trojan, the traitor who leaked the codes. Devil orchestrated Clement's assassination through Manimekala and had been protecting the INA's interests all along.
Devil finally uncovers that Samudra is Agent Trojan, an IPI agent who infiltrated the INA. Samudra had stolen the codes from Nyshadha's room and killed Vijaya when she caught him in the act. He later murdered Rani, who witnessed him covered in blood, and eventually killed Bhuma and Ramana. Samudra reveals Devil's identity to Bracken and takes Nyshadha to the location where Bose is expected to land, planning to betray the INA for a reward from the British. However, Nyshadha, informed by Devil's secret message, diverts Samudra and refuses to cooperate.
In the final showdown, Devil, as Trivarna, arrives at the location, single-handedly taking down Bracken's troops and killing both Bracken and Samudra. Before his death, Bracken learns that Devil, as a British secret agent, will continue to spy on the IPI and other agencies. Bose successfully lands in Mahabalipuram, a new "All Clear" location, and Trivarna, whose true identity remains a secret, resumes his work for the INA, hinting at the possibility of a sequel.
