I wish for this to remain a non-spoiler review. To give away anything of this story, would be a crime in my opinion.
The Greene Tribe live in relative ignorance, generally only aware of their own immediate surroundings, and meagre existence. For them to really consider where they are, is truly beyond them. This is until one of their kind - Roy Complain - decides to investigate beyond his dwellings.
A story can be very powerful when told in the right way. Non-Stop does this in a very well poised and paced manner. Although the book does start slowly, and really does not get going until about a quarter of the way through, the revelations brought upon the reader are truly shocking, with a long lasting effect. I was totally stunned by what Complain discovers. Shortly in, you find out why the book is called 'Non-Stop', and from that point, the shocks keep coming for Complain that turn his whole universe inside-out. He realises that for the whole of his life, and that of his tribe, they have been totally deceived, and that their whole existence is an age-old lie gone horribly wrong.
This is, in my opinion, Aldiss' finest work. Having read the majority of the Sci-Fi Masterworks series, amongst many others, this rates as one of the true greats of the genre. This book will get under your skin, and stay with you for a long, long time.