Tehran-based journalist Arezoo Rahimi travels to the Iranian holy city of Mashhad, driven by the urgency to uncover the truth behind a horrific series of murders committed by a serial killer who preys on local street prostitutes struggling with addiction. Dubbed the "Spider Killer" by the media, this heartless murderer employs a chilling method: he picks up his victims on his motorcycle, takes them to an apartment, and strangles them with their headscarves, before callously discarding their bodies in remote areas on the edges of the city.
In her quest for justice, Arezoo partners with Sharifi, the editor of a regional newspaper. Intriguingly, Sharifi has been contacted by the killer, Saeed Azimi. He has unwittingly become Saeed’s spokesperson as the murderer claims to be on a mission to purify the city in the name of Imam Reza, the eighth Shia Imam. At a poignant moment, Saeed is seen weeping at the Imam's shrine. A veteran of the Iran-Iraq War, he maintains a facade of normalcy with a wife and three children, while working in construction.
Confident in their research, Arezoo and Sharifi devise a plan to trap the killer by enticing him with a staged encounter. Arezoo, masquerading as a sex worker, climbs onto Saeed's motorcycle while Sharifi follows them in a vehicle, but quickly loses track of them in the maze of backstreets. Armed only with a pocket knife and a tape recorder, Arezoo aims to record a confession from Saeed, but her intentions unravel when he discovers her true purpose. In a desperate struggle, she narrowly escapes and rushes to report her findings to the police.
In the aftermath, Saeed is apprehended, but the nightmare is far from over. As the trial approaches, he finds surprising support from the public. When presented with the chance to claim insanity, he vehemently defends his actions, attributing them to his fervent devotion to the Imam and God, stating, > "I am only crazy about the eighth Imam." During an intense interview in prison, Arezoo confronts him, where he chillingly admits to the murder of 16 women, asserting that she would have been his next victim.
The verdict arrives with Saeed receiving a grim sentence of 100 lashes and death. In a twist of fate, while awaiting execution, he is visited by his father-in-law and lawyer, who assure him of a last-minute rescue. However, when the moment of his execution comes, the anticipated escape fails to materialize, resulting in Saeed facing execution by hanging in a moment of raw panic.
As Arezoo boards a bus back to Tehran after saying her farewells to Sharifi, she reflects on the horrors she's witnessed. While journeying home, she reviews video evidence from the case, lingering on a disturbing interview with Saeed's teenage son, Ali. In a chilling display, Ali proudly reenacts how his father overpowered and choked his victims, using his younger sister to demonstrate the gruesome acts, leaving Arezoo haunted by the cycle of violence that endures even beyond Saeed’s capture.
