Elena and Ernesto are a couple who are bitterly torn apart by the death of their only child. Three years later they have both somehow managed to start a new life. Until destiny forces them to reconnect and understand, they never really stopped loving one another other.

Prime numbers are divisible only by one and themselves. These numbers are solitary and incomprehensible to others. Alice and Mattia are both "prime", both haunted by the tragedies that have marked them in childhood: a skiing accident for Alice which has caused a defect in her leg, and the loss of his twin sister for Matthew.

Summer 1943, Piedmont, Italy. Milton loves Fulvia who plays with his love: she only likes the depth of his thought and the letters he writes to her. One year later, Milton has joined the Resistance and fights beside other partisans. He learns during a conversation that Fulvia was secretly in love with his best friend Giorgio, a partisan like him. Milton decides to go find Giorgio in the Langhe region of Italy with all its misty hills...But Giorgio has just been arrested by the Fascists.

Italy, summer of 1987. The infamous "Killer" gang is initiating two new members, and it's going to be mess