Maisie finds herself coming to the aid of her friend Professor Tobin. The professor has travelled all over the world, collecting strange and wonderful objects. Now he plans to donate his collection to the British Museum – including a rare and valuable tribal mask from the Amazonian rainforest. But when a thief breaks into the boarding house and steals the Feathered Mask, leaving only a single feather, Maisie realizes she has a new mystery on her hands.