After failing to sell any of his paintings at a funfair, an artist begins to tell his granddaughter about the origins of his paintings. We are then transported on board a British convict ship headed for New South Wales where we meet six convicts; Toby Nelson (a young boy), Polly Nelson (Toby's older sister), "Dipper" Davey (an old pick-pocket), Big George (a Blacksmith), Jack Doolan (a highwayman), and "Silly Billy" (a village idiot). The five men are selected to do hard labor for the military - under the supervision of Sergeant "Bully" Langden and Corporal "Weasel" Wesley - while Polly (secretly in love with Big George) is sent to work as a servant girl at the governor's home. Toby in the meantime saves a baby koala after Bully and Weasel kill it's mother while hunting, and keeps it as a pet, naming it Yo-Yo. But when Bully's continuing abuse of the convicts ends in old Dipper's death, Jack Doolan, fearing a similar fate, escapes and Big George is thrown in jail for helping him to escape. Left with only Yo-Yo to keep him company, Toby sets out to find Jack Doolan in order to free Big George. Along the way he meets an Aboriginee boy name Whooranga, who has a strange power over animals. Together with Jack Doolan, our three heroes plan a daring rescue attempt for Big George, only to be faced with making the ultimate sacrifice upon carrying it out...