I was just thinking about this book the other day! The main plot is delightfully simple, yet surprisingly chaotic. Mr. Popper is a quiet house painter who dreams of polar exploration. Out of the blue, he receives a penguin as a gift from an Antarctic admiral. That single penguin, Captain Cook, is the catalyst for everything. The chaos truly begins when the penguin gets lonely, so Mr. Popper acquires a second penguin from the zoo (Greta), and they promptly start a family. Before you know it, the Popper household is overrun with a dozen penguins.
This forces Mr. Popper to get creative. He turns their basement into a winter wonderland with ice and a freezing apparatus. The financial strain of feeding them all leads to the brilliant, absurd idea of training the penguins for a vaudeville act. The 'Popper's Performing Penguins' act becomes a nationwide sensation, which is the core of the book's middle section—a series of hilarious mishaps as they tour the country. The plot resolves when the penguins become too famous and the strain of touring is too much; a happy ending sees the whole troupe (and Mr. Popper himself) sailing off to the Arctic for a new life. It's a classic underdog story where a mundane man's life is turned upside down by a flock of birds, pushing him to become an impromptu entrepreneur and adventurer.