This novella (132pp) is set many years into the future. Humans are divided into two classes - the outside people and the inside people. The inside people never go out into the world, which is a deadened, crumbling wasteland, giving way to occasional thick jungle. Everything they need is delivered via the tubes, and if they want to see someone, they send a hologram of themselves and receive their own visitors in the same way. Uncumber is a young girl barely out of adolescence, and she's always been different, often refusing the calmant tablets the insiders use to regulate their moods. Uncumber wants to feel her moods, and though she is afraid, she is also very curious about the outsiders.
One day Uncumber gets a chance to see how outside people live. She lives among them for a while, courtesy of a bewildered outsider and his family, who accept charge of Uncumber rather ungraciously (and entertaining use is made of their different languages), but when she wants to go back to her privileged inside existence things don't quite fall into place and Uncumber falls into the hands of a group of what appear to be French brigands.
I found the plot rather predictable, though there is plenty of playful humour in the writing and it is not a boring read. Rather better than Frayn's other venture into the future, Sweet Dreams, this has a point to make, if a rather laboured one. Be careful what you wish for.