Doctor Dolittle establishes a swallow mail service for the animals when he discovers that they have their own way of writing.

Set on the West Coast of Africa, the book follows Doctor Dolittle who helps to capture a slave trader's ship, then organizes the postal service of a small African kingdom, Fantippo, ruled over by King Koko. He later discovers a hidden island populated by prehistoric creatures and attempts to make contact with them, with numerous dangers and difficulties waiting in line.

In writing the story of our adventures in the Moon I, Thomas Stubbins, secretary to John Dolittle, M.D. (and son of Jacob Stubbins, the cobbler of Puddleby-on-the-Marsh), find myself greatly puzzled. It is not an easy task, remembering day by day and hour by hour those crowded and exciting weeks. It is true I made many notes for the Doctor, books full of them. But that information was nearly all of a highly scientific kind. And I feel that I should tell the story here not for the scientist so much as for the general reader. And it is in that I am perplexed. For the story could be told in many ways. People are so different in what they want to know about a voyage. I had thought at one time Jip could help me; and after reading him some chapters as I had first set them down I asked for his opinion. I discovered he was mostly interested in whether we had seen any rats in the Moon. I found I could not tell him. I didn’t remember seeing any; and yet I am sure there must have been some—or some sort of creature like a rat. Then I asked Gub-Gub. And what he was chiefly concerned to hear was the kind of vegetables we had fed on. (Dab-Dab snorted at me for my pains and said I should have known better than to ask him.) I tried my mother. She wanted to know how we had managed when our underwear wore out—and a whole lot of other matters about our living conditions, hardly any of which I could answer. Next I went to Matthew Mugg. And the things he wanted to learn were worse than either my mother’s or Jip’s: Were there any shops in the Moon? What were the dogs and cats like? The good Cats’-meat-Man seemed to have imagined it a place not very different from Puddleby or the East End of London. No, trying to get at what most people wanted to read concerning the Moon did not bring me much profit. I couldn’t seem to tell them any of the things they were most anxious to know. It reminded me of the first time I had come to the Doctor’s house, hoping to be hired as his assistant, and dear old Polynesia the parrot had questioned me. “Are you a good noticer?” she had asked. I had always thought I was—pretty good anyhow. But now I felt I had been a very poor noticer. For it seemed I hadn’t noticed any of the things I should have done to make the story of our voyage interesting to the ordinary public. The trouble was of course attention. Human attention is like butter: you can only spread it so thin and no thinner. If you try to spread it over too many things at once you just don’t remember them. And certainly during all our waking hours upon the Moon there was so much for our ears and eyes and minds to take in it is a wonder, I often think, that any clear memories at all remain. The one who could have been of most help to me in writing my impressions of the Moon was Jamaro Bumblelily, the giant moth who carried us there. But as he was nowhere near me when I set to work upon this book I decided I had better not consider the particular wishes of Jip, Gub-Gub, my mother, Matthew or any one else, but set the story down in my own way. Clearly the tale must be in any case an imperfect, incomplete one. And the only thing to do is to go forward with it, step by step, to the best of my recollection, from where the great insect hovered, with our beating hearts pressed close against his broad back, over the near and glowing landscape of the Moon.

Doctor Dolittle heads for the high seas in perhaps the most amazing adventure ever experienced by man or animal. Told by nine-and-a-half-year-old Tommy Stubbins, crewman and future naturalist, the voyages of Doctor Dolittle and his company lead them to Spidermonkey Island. Along with his faithful friends, Polynesia the parrot and Chee-Chee the monkey, Doctor Dolittle survives a perilous shipwreck and lands on the mysterious floating island. There he meets the wondrous Great Glass See Snail who holds the key to the greatest mystery of all.

Doctor Dolittle's Post Office is the third of Hugh Lofting's Doctor Dolittle books. Set on the West Coast of Africa, the book follows the episodic format of most other books in the series.

On a succession of evenings, the animals settle into Doctor Dolittle's kitchen to hear Gub-Gub the pig read parts of his book on food.

After a three-year voyage, Doctor Dolittle sets his Puddlesby garden to rights and and makes it the site of an animal-run zoo. With the help of his friends, he also tackles a mystery at nearby Moorsden Manor.