The book opens with an elderly lady being knocked down by a mugger, resulting in her breaking her hip and having to stay temporarily with her daughter until she is recovered. This one apparently small incident triggers all manner of other, much more major events that would not otherwise have happened. Things like a wife discovering that her husband is having an affair, a woman moving from London to the provinces and re-marrying, an Eastern European immigrant learning English sufficiently well that he is able to start a career in accountancy (his true profession) in the UK.
Lively loosely links this cascade effect to chaos theory via a quotation right at the beginning of the book. But, as always with Lively, it is the story that is paramount, and she really is a master storyteller. To read her prose is, I think, like listening to someone speak, with the plot unfolding apparently effortlessly through dialogue and sharp, witty observation.
If you are looking for a good holiday read, this book would be ideal. Alternatively you could read it (as I did) in snatched moments between work and childcare duties.
And, if you enjoy How it all Began, I would recommend another Penelope Lively novel—Making it Up—in which Lively takes significant decision points in her own life and considers what might have been had she made different decisions from the ones she actually did. This book is not so much about how it all began, but more about how it all continues—and ends.