Mr and Mrs Twit are mean - in fact very mean - to each other; they play tricks on each other, which appeal to your sense of humour. They are cruel to birds - who hates birds? They're also horrible to Muggle-Wumps, a friendly group of monkeys. However, when the Roly-Poly Bird arrives the birds and Muggle-Wumps manage to turn the tables on the Twits (quite literally).
Roald Dahl's story is clever and cleverly funny. I couldn't think of a child between six and nine not enjoying this - or any of his other 'classics', 'Fantastic Mr. Fox', 'The BFG', and 'Charlie and the Chocolate Factory' to name but a few. This is a very good child's book, written by the best children's author of all time - is there any better? - supported by superb illustrations by Quentin Blake, so you can really hate the Twits.