Raman Raghav was a well-known serial killer, having terrorized the city with his brutal crimes in the 60s. In 2015, a similar psychopath, named Ramanna (Nawazuddin Siddiqui), begins to operate in Mumbai. The tabloids label him as 'Stoneman' since the first few murders he commits with a stone. On one particular night, a cocaine addict comes looking for his supply and comes across a crime scene. A few days later, Ramanna is apprehended by police and held for interrogation. The man who questions him is revealed to be the same cocaine addict, Officer Raghavan (Vicky Kaushal).

The police detain Ramanna for some time, harassing, torturing and starving him until he is finally able to escape with the help of a gang of young boys. Ramanna continues his killing spree and hunts for a new victim. He finds his next prey in his sister Lakshmi, a poor woman living with her husband and son. He enters her home under the pretext of needing a meal but soon ends up in an argument with her husband. Enraged, he storms off, only to return later on and kill the whole family. The police arrive at the scene, and Raghavan finds an old picture of Ramanna and his sister, effectively confirming him as being involved in the crime.

Meanwhile, Raghavan is a deeply troubled person, suffering from anger issues and drug addiction. He is in a relationship with a woman named Smrutika (Simy), but he refuses to commit or be faithful to her. Their relationship is dark and abusive and Raghavan has pressured Simy into having more than one abortion. Raghavan becomes obsessed with the murders and catching Ramanna, to the point where he finds himself becoming consumed by cocaine. Ramanna has been looking for his soul mate, his other half, and realizes that he has found him in Raghavan. The pair engages in a cat-and-mouse game in which Ramanna seems to have the upper hand as Raghavan spirals out of control.

Ramanna starts stalking Simy and ends up murdering her maid. Simy is overwhelmed by Raghavan's drug use, mood swings and callous attitude. After a cocaine-fueled night, Raghavan brings another girl named Ankita to Simy's house. He is forced to take Viagra due to the effects of his earlier drug use but is still unable to perform. Ankita mocks him, causing Raghavan to strike her. In a fit of rage, he begins searching for the stash of drugs he usually keeps in Simy's house, but he is unable to find it. He confronts Simy and the pair engage in a heated argument, which results in Simy's accidental death. Ankita tries to call for help but Raghavan threatens her and locks her inside a room. He then clears away all drug paraphernalia and sets the scene so as to incriminate Ramanna for the murder of Simy.

Ramanna surrenders to Raghavan and tells him that they are each other's missing half, the two sides of the same coin. Raghavan realizes this and decides to go after the only witness to Simy's murder, Ankita.