Mumbai is in a state of chaos. All traffic signals across the entire city have stopped working. Shahwaz Ali Mirza, head of the Maharashtra Anti-Terrorism Squad, receives an anonymous email claiming it to be a Distributed Denial of Service (DDoS) attack. He quickly puts together a crack team that includes his protege, IG Cybercrime Vikrant Singh, and gets to work trawling the dark web for more information on this mystery attack. However, a move to bring forward the hacker backfires, leading to a second, deadlier attack on Mumbai's lifeline, the railway system. It is their first brush with cyberterrorism: a zero-day vulnerability in the Indian government's system that could bring the country to its knees. Racing against time and investigating a case unlike any other, in Zero Day, Mirza and Vikrant face the most dangerous mission of their lives.