Six-year-old Billie feels left behind by her teenage sister and ignored by her mother, who is singlehandedly raising her daughters and trying to keep the household together. With her fierce desire for independence and habit of pushing the boundaries, Billie finds herself in situations that, despite her best attempts at being a ‘grown-up’, she is not quite ready for. But she and her best friend Milla are finally given the chance to prove they’re just as responsible as the ‘big kids’: they can walk to the shops alone, without a babysitter. Along the way, however, their dark and wild imaginings suddenly become a reality when one of their games puts Milla in danger. It is up to Billie to rescue her friend before it’s too late.