Mr. and Mrs. Twit are a hateful and spiteful couple who take perverse pleasure in playing cruel jokes on each other. Their pranks range from Mrs. Twit feeding her husband worms and Mr. Twit tormenting his wife into thinking she is afflicted by a mysterious "shrinking disease." There is seemingly no end to the cruelty this couple has towards each other and unfortunately, those around them fare no better.
The Twits, who were once retired circus trainers, continuously abuse a family of monkeys (known as the Muggle-Wumps) that are in their care. Spurred on by their desire to create the first "upside-down monkey circus," the Twits force the Muggle-Wumps to endure grueling training every day by threatening to beat them if they do not submit.
Furthermore, The Twits use a strong sticky glue called "hugtight" to trap birds so they can make them into a pie. The Muggle-Wumps, continuously try to warn birds not to land on a tree that has been coated with "hugtight" but are unable to communicate with the English-speaking birds as the Muggle-Wumps are originally from Africa. With the help of a friend, the Roly-Poly bird, the Muggle-Wumps manage to warn the birds about the Twits plot despite Mr Twit's attempt to fool the birds.
Sick of the Twits and their schemes, the Muggle-Wumps come up with a plan to get rid of them for good. Using the "hugtight," the birds glue the couple's furniture on the ceiling to trick the couple into thinking that they are upside down when they first walk into their house. The birds also drop glue on the Twits's heads. When the Twits return home, they are terrified upon believing they are upside down and accidentally glue themselves to the ground when they stand on their heads so they are "right way up." Because of all the pressure that is put on their heads, the Twits begin shrinking until they shrink into nothing. All the animals rejoice and the Muggle Wumps escape.
