The Story of a Gun

Younes stumbles upon a murder case in which an entire family has been killed during a birthday celebration.

Younes stumbles upon other murders where the common thread is the blood relation of the victims to one another, and he realizes they are dealing with a serial killer.

Younes obtains clues that relate to the contaminated vaccines case in which Amir-Ali Nadaf had committed murders.

A new murder is reported, and the handling of the case is transferred from Younes to Mohsen Mowla, but Amir-Ali Nadaf prevents Younes from leaving the investigation.

Amir-Ali Nadaf reveals a big secret to Younes: a case from the past that plagued Nadaf and changed the course of his life.

Nadaf recounts to Younes the story of the setup he was framed with years ago. Simultaneously, Sohrab, the only child of the person who saw the face of the serial killer, returns to Iran because his father is still alive.

Nadaf attempts to get closer to Bahar, who is one of the survivors of the past murders, while Younes and Mohsen also obtain new clues.

Younes and Mohsen discover common threads among the victims, while Nadaf remains haunted by a lost past.

Younes, now almost certain that Amirali Nadaf is not the killer, enlists his help to solve the mystery, while Siavash also offers to assist, unaware that Amjad is the one responsible for his father's death.

Younes, Amirali Nadaf, and Siavash join forces to track down one of the individuals involved in the contaminated vaccines case.