What do existential elevators, sentient mattresses, paranoid androids, humans and other aliens have in common? For one thing, they want answers. The fact (yes fact) that there are no answers (except, perhaps, for "42") causes some humans (and other aliens) to face this empty madness we call life with Sisyphus-like defiance. Others choose to sulk or skulk or annihilate themselves. Another thing these creatures have in common is that they are all born mad, "and some remain so". One is never alone with a rubber duck explores the premise that Douglas Adams's Hitchhiker Series is not merely characterised by light-hearted comedy, but is underpinned by intricate philosophical ideas, especially those of twentieth century Existentialism and the related notion of absurdity. It also investigates the interlaced functions of Adams's fantasy and landscapes of alterity, and considers the ambiguous concept of madness as subjective reality. Concepts related to alterity, such as simulation, the structure of reality, dreaming and parallel universes, are investigated as part of Adams's fantastic story space.In a science-fictional sense, Adams's aliens satirise the human condition and the monstrosities that lurk at the heart of twentieth century society.