Marlowe and Harper share a bond deeper than most sisters, shaped by the loss of their mother in childhood. For Harper, living with what she calls the Up syndrome and gifted with an endless capacity for wonder, Marlowe and she are connected by an invisible thread, like the hum that connects all things. For Marlowe, they are bound by her fierce determination to keep Harper, born with a congenital heart disorder, alive. Now 25, Marlowe is living her own life abroad, pursuing her studies of a rare species of butterfly secure in the knowledge Harper's happiness is complete, having found love with boyfriend, Louis. But then she receives the devastating call that Harper's heart is failing. She needs a heart transplant but is denied one by the medical establishment because she is living with a disability. Marlowe rushes to her childhood home in Hong Kong to be by Harper's side and soon has to answer the question.