The plot concerns a young boy, Jon, who accidentally 'falls' to Earth through the titular portal. Accustomed as he is to his utopian, pacifistic, vegetarian, and money-free world, Jon's bewildered attempts to connect with the petty grievances of Earthlings serve as foil to the reader; and while there is some action and suspense, the main tension of the story comes from the sympathetic family that shelters Jon being forced to confront their own complacency towards the injustices of their world while trying to make Jon understand that this is just how life is — but of course, for Jon and his world, it isn't.
Considering the state of the world in 1965, the book sends a clear message that the future, and we humans, can, and must, be better.
This book provides examples of:
- Aliens Speaking English: Averted, though he is a very quick learner.
- Identity Amnesia: After falling and hitting his head, Jon forgets who he is and where he came from.
- Human Aliens: Jon.
- In Which a Trope Is Described: Each chapter title is a sentence beginning with "He..." as in "He is Lost and Found" and "He Gains a Home."
- Living Lie Detector: Due to his telepathy. It becomes a problem later on.
- Telepathy: Jon is able to read others' minds as well as animals.