A prime number is inherently a solitary thing: it can only be divided by itself, or by one; it never truly fits with another. Alice and Mattia also move on their own axes, alone with their personal tragedies. As a child Alice's overbearing father drove her first to a terrible skiing accident, and then to anorexia. When she meets Mattia she recognises 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 to a party, and when he returned, she was nowhere to be found. These two irreversible episodes mark Alice and Mattia's lives for ever, and as they grow into adulthood their destinies seem irrevocably intertwined. But then a chance sighting of a woman who could be Mattia's sister forces a lifetime of secret emotion to the surface. A meditation on loneliness and love, ''The Solitude of Prime Numbers'' asks, can we ever truly be whole when we're in love with another?