Sophie can't walk or talk, but behind her disability hides a keen intelligence. Living on The Styx River cattle station with her father and a nanny, Sophie is acutely aware that she is a non-person. She feels as voiceless and isolated as the wallabies of The Wall, an eerie wilderness of basalt lava tubes forming a natural stone labyrinth that protects its remote lushness from anyone foolish enough to wander in.

Sophie's mother Rose, as The Wall's indigenous custodian, taught Sophie its secrets. When a bitter divorce forces Rose to leave, Sophie is powerless to stop her grazier father from taking custody of both Sophie and The Wall.

Advances in computer technology enable Sophie to communicate, a fact she keeps secret from her father and his "cronies". In Sophie's presence, unaware she understands everything, they plot to develop a multi-million dollar tourist resort in The Wall. The development will only go ahead if the rare wallabies are already extinct, so they hire roo shooters to help nature along.

In desperation Sophie writes Silent Scream, an anonymous blog that reveals the threat. When an environmental study team commissioned to find the wallabies goes missing, the rescuers appeal to Silent Scream for help. Raising awareness is one thing, but how can one impossibly disabled girl who can't help herself, help save the lives of others?