The Twits are nasty, old people who have nothing better to do than play mean tricks on each other. They look just as mean as they behave. Mr. Twit has a long, spiky beard full of food droppings, and Mrs. Twit has an ugly face and a glass eyeball. She carries a walking stick not because she is feeble, but so she can hit animals and small children in her path.
One day, Mrs. Twit surprises Mr. Twit by putting her glass eye in his beer. Mr. Twit retaliates by putting a frog in her bed. After Mrs. Twit puts worms in his spaghetti, Mr. Twit plays the meanest trick of all. Every night after Mrs. Twit goes to bed, he glues small circles of wood to the bottom of her walking stick and chair. She doesn’t notice until the walking stick has reached her shoulders, and then Mr. Twit convinces her that she has the shrinks. The only cure for the shrinks is to stretch, so Mr. Twit ties hundreds of helium balloons to her arms. When she is aloft, he cuts the rope holding her to the ground, and Mrs. Twit flies away. She comes back to earth by biting the balloons off one at a time until she slowly floats downward.
Their house fits their nasty personalities perfectly. They planted the garden with thistles to keep kids out, and didn’t build any windows into the house so no one can see in. They keep four monkeys in a cage that Mr. Twit is training to be the world’s first upside-down monkey circus. He makes them practice standing on their heads for six hours a day, which makes the monkeys very ill. They also have a big dead tree that Mr. Twit paints with glue once a week to catch birds for pie. The monkeys always try to warn the birds, but the monkeys are from Africa, and the English-speaking birds don’t understand them.
Roly-Poly bird flies in from Africa to find his monkey friends, the Muggle-Wump family, locked in the Twits’ cage. The next day is bird pie day, so the monkeys shout a warning to Roly-Poly. They enlist his help to keep the other birds form getting stuck. When Mr. Twit finds all the birds perched on the monkey cage the next morning instead of the tree, he is furious. Determined to have bird pie, he paints glue on the monkey cage and the tree. The next morning all the birds are perched on the house, thanks to Roly-Poly’s warnings.
The Twits go to town to buy a gun. Roly-Poly steals the monkey cage key from the shed and frees the Muggle-Wumps. They round up all the birds to help give the Twits a dose of their own medicine. They quickly cover the Twits’ ceiling with glue. They attach the carpet and all the furniture to the ceiling, and then turn the pictures upside down. When the Twits are walking back, two ravens put glue on the Twits’ heads.
Mr. and Mrs. Twit enter their home and believe themselves to be upside-down. Unable to right themselves easily, they try standing on their heads. The glue attaches them to the floor. The Twits get the shrinks from standing on their heads for so long, and keep shrinking until they disappear.
The Muggle-Wumps build a tree house in the woods that all the birds guard. Roly-Poly flies them back to Africa, one at a time.