The Mennyms had weathered the (false) alarm of a visit from Aunt Kate's nephew, in fact Magnus has receiving a life-interest in their long-time home, but it is soon followed by a much more distinct threat to their safety: the wrecking ball. They receive a notice that their house, their whole quiet street in fact, must make way for a new road.
Help arrives, with a supernatural nudge, in the form of a relative of Aunt Kate's: Albert. Albert, a young man, is drawn into the Mennym's small world and becomes enchanted by them. He initiates a plan to help save their home, and when the outside world's curiosity threatens them, he brings them to a remote country house.
What really impresses me about this series is that Waugh really gave a lot of thought to the many complications that a living doll family could face. The simple solutions to their problems always have a catch that get picked up on. There are thorny issues like a modern bureaucracy catching on to the fact that the same 'man' has leased a property for 60+ years, for example.
The psychology of the Mennyms is complex as well. Its pointed out that for years, decades, at a time the Mennyms follow the little patterns of their pretends. They area static, but then a single change in their daily lives leads to experience and 'growing' up. Appleby and Pilbeam in particular face all of the pangs of being on the cusp of adulthood, forever. The danger of Albert, a person, being involved in the life of the Mennyms after the crisis of the outside world ends, leads to the inevitable conclusion.