Having expanded their treehouse to an astonishing 39 stories, Andy and Terry describe the invention of their Once-Upon-a-Time machine, which they have designed to write and illustrate their stories for them.

Andy and Terry recount the story of how they first met, an adventure marked by emergency shark operations, giant storms, and wooden pirate heads.

When Andy develops a severe toothache that prevents him from writing new jokes for their book, Terry uses their fantastical treehouse's money-making machine funds to purchase a Joke Writer 2000, with unexpected results.

An overly alarmed Chester the cat predicts a gruesome fate for the pets in the Monroe household when a writer of juvenile horror fiction and his bird companion stay overnight.

"When Tuesday McGillycuddy and her beloved dog Baxterr discover that Tuesday's mother--the famous author Serendipity Smith--has gone missing, they set out on a magical adventure. In their quest to find Serendipity, they discover the mysterious and unpredictable place that stories come from"-- Provided by publisher.

Follows the efforts of Andy and Terry to meet a latest book deadline despite numerous distractions in their sophisticated tree house home, including flying cats, giant bananas, and sea monsters pretending to be mermaids.