A prime number is a solitary thing: it can only be divided by itself, or by one; it never truly fits with another. Alice and Mattia are also alone with their personal tragedies. As a child Alice's overbearing father drove her first to a skiing accident, and then to anorexia. When she meets Mattia she recognizes a kindred spirit, and Mattia reveals to Alice his terrible secret: that as a boy he abandoned his mentally disabled twin sister in a park to go a party, and when he returned, she was nowhere to be found.