If I Ran the Zoo opens with the book's protagonist and narrator, Gerald McGrew, arriving at a zoo alone. The young boy stands before a lion cage. In it, a single lion is lying down and looking content. The lion's expression is similar to that of the zookeeper, who stands next to the cage with his hands in his pockets.
Gerald comments that it is a pretty good zoo, and that the man who runs it seems proud. However, Gerald begins to speculate about what fascinating imagined creatures he would bring to the zoo if he ran it.
Narrating in anapestic tetrameter, Gerald lists the animals he would capture from far-flung regions of the world. Some regions are actual places, such as North Dakota, while others are invented, such as the Island of Gwark.
The animals Gerald says he would capture include a ten-footed lion, an elephant-cat, a Flustard (who eats only mustard and custard), a goat-dog-squirrel hybrid called the Joat, a family of deer with their antlers knotted together, a cave-dweller called the Natch, and finally the world's largest bird, a Fizza-ma-Wizza-ma-Dill.
Throughout his narration, Gerald speculates that the public would react in astonishment to every new creature he brings back to the McGrew Zoo. However, the story returns at the end to reality: Gerald is still standing before the lion exhibit and the actual zookeeper. The book ends with Gerald commenting that these are the changes he would make if he ran the zoo.
