Red Hellion lives opposite the creepy, tightly locked Torrington Park, or 'Viper's Green' as is more commonly known, but has never been inside. Walking home from school one day, he sees a boy crouching at the base of the gates, trying to break into the park. The boy, Max, seems intent on exploring the park, and tells Red he is looking for snakes. Puzzled and curious, Red follows him in, expecting to find nothing, but what he does find is well beyond what he could ever have imagined, and plunges him into terrible danger.