Once upon a time, abbess Maria and sisters Arsinoe, Ourania, Agathoniki and Chrysovalanto lived calmly and peacefully in the monastery of Agia Theodouli. Until they discover a man's corpse in the sacristy and their lives turn into hell... The investigation of the case is undertaken by the experienced police officer, Miltiadis Sofianos, and his rookie assistant, Hector Tsakalakis, with orders to conduct the investigation in strict secrecy. Sofianos and Tsakalakis have the nuns as prime suspects and interrogate them. Strangely enough, none knew the victim and none saw anything suspicious on the night of the murder. However, evidence soon comes to light that indicates the opposite: one of the nuns secretly put the victim in the convent and then murdered him.

The arrest of the abbess causes fear and disturbance to the rest of the nuns, who begin to doubt her innocence. The police pressure the abbess to admit her guilt, but she denies any involvement in the murder. Fearing possible condemnation, she decides to take fate into her own hands and begins a secret investigation with the aim of solving the murder herself. At the same time, Karaiskos and Stamatis also start an investigation to find out what happened to Petros. Meanwhile, Sofianos and Tsakalakis discover a new clue that proves that one of the nuns gave a false testimony. On the night of the murder, she left her cell and had a suspicious date outside the monastery.

A threat from the past terrifies Abbess Maria, causing her to flee into the night and disappear. Sister Arsinoe is in despair, as -on the one hand- she searches in vain for the abbess and on the other she is called to save Chrysovalanto from Karaiskos who has kidnapped her. Sofianos and Tsakalakis are informed by Father Fanourios about the mysterious relationship that a certain nun had with Petros Angelou, and they too look for the abbess for answers. Dimitris Kalogirou is also following in the footsteps of the abbess, trying to kill her...

The nuns have now learned that Abbess Maria is dead. They mourn her loss full of questions. Sister Arsinoe decides to tell the other nuns everything about the secret investigations of the abbess that led to her death. Sofianos and Tsakalakis try to "unmask" Dimitris, who is behind the murder, while Karaiskos seems to have escaped them. As the nuns and Father Fanourios mourn the loss of the abbess, one of the nuns, affected by the tragic outcome, visits the police station and confesses to the murder of Petros Angelos. Who is she?