On a school trip to the Royal Swan Natural History Museum, Ava finds herself at the centre of a heist: hundreds of invaluable bird of paradise skins from the Alfred Wallace Collection have been stolen! When she discovers a large jet-black feather, she's sure it's a clue, but the police won't listen to her. Slipping away, she takes the case to the Twitchers in an investigation that leads them to the bizarre and sinister world of fly-fishing, and to a mysterious girl with a pet Raven called Caliban and a dangerous ring of thieves and smugglers.