Somewhere in Northern Europe, at the end of 19th Century.
An isolated fortress stands on top of a hill. There's only one prisoner who's still being held in this desolated place: the man must have been found guilty of unspeakable acts to have been imprisoned in such a horrendous piace. He has been in the fortress for many years and nobody is allowed to approach him. No one can remember what atrocious crime the man committed.
A new captain arrives at the fortress accompanied by his wife, Adele, and his seven-year-old son Ottaviano. Adele is a young middle-class woman. She's kind, reserved and loves playing the piano. Across the fortress courtyard the sound of a violin can be heard: the melody comes from the prisoner's cell. Adele is captivated by the music and replies to the sound by playing her piano.
Both her life, and that of the prisoner, is shaken by this new contact, and the prisoner once more becomes aware of a world outside his cell. The piano and the violin sound as if they are played by the same hands: the woman and the man, even though they've never met, become one to the point that their relationship begins to involve and upset Adele's family. Little Ottaviano is afraid that the gigantic man - though the only gigantic thing about him is his shadow - is going to steal his mother, and the captain suddenly feels as utterly alone as the prisoner. Adele falls seriously ill, consumed by an isolation which is driving her crazy.
She and the prisoner will die, one soon after the other. The captain and his son willleave from the fortress: the place is now completely lifeless and will be pulled down very soon. The shy watchman has feelings for her, she stops seeing him. However, Paolo's investigations continue and he even starts suspecting his own colleagues, including his new partner who is now on night shifts with him because of the new regulations established after the murder.
Paolo is now forced to be accompanied by silent Salvatore Russo, who is obsessed by a jealous and self-destructive wife. One night, after Salvatore has run home to calm his suicidal wife down, Paolo meets Dimitri. Everything is more confusing than ever and everyone involved seems to hide the real truth. The watchman feels alone in the heart of a mystery that nobody wants to unveil. Who shot Catellota that night?