On a raft moored on the riverbank, Tom Sawyer tells his friend, Huck Finn, he is jealous that his friend, Becky Thatcher, has gone on an adventure to St. Louis, Missouri. Soon, Tom realizes that he and Huck can create their own adventure by finding a hot-air balloon on display in St. Louis. There, the boys meet Becky, learn that Haley’s Comet will appear shortly, and hear author Mark Twain tell the crowd that he will embark on a voyage aboard the hot-air balloon “to meet” the comet. Since the comet appeared when he was born, Mark Twain plans to depart with its return. Meanwhile, Tom and Huck stow aboard the balloon, and Tom starts the engine, gloating that he will have stories to tell Becky, only to find that Becky has also climbed aboard. While Twain is pleased to see unexpected company, Huck wonders how they will return to Earth. However, the writer says the goal is to find the comet, not to go home. As Twain shows them a map pinpointing the comet’s trajectory, he sees Homer, the boys’s frog, and proceeds to tell them his story about the famous frog of Calaveras County, in which his friend Jim Smiley caught a creature, and spent three months teaching it to jump great distances. One day, Smiley bet a miner that his frog could out jump any frog in Calaveras County. Smiley found the miner a competing frog so that he could place a forty-dollar bet. After the miner’s frog easily won, Smiley discovered that his frog’s stomach was weighed down with tools, and other miscellaneous detritus. Smiley searched for the miner that swindled him, but never found him. Soon, the friends ask Twain to land and remove Becky as a ruse to leave the balloon, but Twain refuses, and the journey continues. Tom and Huck explore the balloon to find another escape. They discover a teleporter machine onboard called the Indexivator, which transports them to Twain’s study. Becky follows, and all the youngsters listen to Twain’s reading of The Diary of Adam and Eve. In the story, God created a beautiful garden, and formed Adam out of a ball of clay. Adam had no cares, and for fun, he rode over a waterfall. Soon, he met Eve, but he was frightened by her and ran away. Eve kept a diary of Adam’s activities. He avoided her, and she tried unsuccessfully to pick apples from the forbidden tree as a token of friendship. Eve occupied her time naming objects in the garden. Adam was not impressed, but eventually they spoke. When Eve’s presence became too suffocating, Adam kept his distance. Eve made friends with other animals in the garden such as the snake. Meanwhile, Adam cautioned Eve that eating forbidden apples would bring death into the world. Eve thought this would be a good thing for lions, tigers and vultures. Once Eve ate the apple the world became sadder and darker. Adam escaped so as not to be touched by the troubles brought on by the apple. Back on the balloon, the travellers encounter a storm. Soon, the Indexivator introduces them to a mysterious stranger, a headless figure carrying a mask on a stick. He shows them a miniature castle, and gives them clay to create people to inhabit it. The mysterious stranger brings the clay people to life, but as they start to fight, he destroys them. Then, he causes an earthquake that wipes out the rest of the clay people. As the mysterious stranger’s face metamorphoses into a skull, he proclaims life is only a vision. Twain and the friends escape by the Indexivator. While they stand around perplexed, Twain leaves through an Indexivator door marked “The Damned Human Race.” The next day, the friends chain Twain to a chair in hopes of hijacking the balloon. However, they see a liberated Twain peering through his telescope on deck, and learn that their mutinous act was an illusion courtesy of the Indexivator. Soon, they reach the Great Sphinx in Egypt, and Twain drops anchor on the monument. Tom secretly rigs a hatchet to damage the ship’s power panel for a second hijack attempt. As the friends wait, they persuade Twain to read more of Adam and Eve’s diary. In the story, Adam was content to live outside the garden, although the world changed because of Eve. Adam and Eve lived together, and Adam earned a living being a superintendent. Eve discovered how to build a fire, and gave birth to Cain. However, Adam was confused, and, at first, thought that Cain and subsequent offspring were fish. Later, he came to appreciate his children, and he and Eve grew old together. Adam remarked that Eve represented Eden wherever she might be. At the story’s end, Twain comments that he is old and tired and wishes to be with his wife, Lilly. When the children say they do not want to die, Twain tells them once he finds the comet, the ship will be theirs. They are relieved, but then the ship’s power panel is destroyed thanks to Tom’s efforts. Before the ship blows up, Huck directs Homer to jump through a porthole to the deck, and push the emergency control button. After Homer saves them, they recognize the comet high in the sky. Twain orders the friends to throw his heaviest possessions overboard to lighten the load so that the balloon can reach the comet. Twain activates the ship’s thrusters, and they float into the comet’s atmosphere. Suddenly, a Mark Twain clone appears, representing Twain’s darker side. Only Becky realizes the clone has been aboard all along. The boys want to know which Twain is the real one. However, Mark Twain explains that the second man is part of him, and he cannot make the journey without him. Both Twains take turns giving advice to the children. Then, their bodies fuse together, and hitch a ride with Haley’s comet. Twain tells Becky he is on his way to Eden. As the friends steer the balloon back to civilization, Twain’s voice, emanating from the cosmos, tells them that laughter is the human race’s only effective weapon. With it, nothing else can stand. Tom and Huck figure that they can write about their adventure since Twain’s life is over, and the torch has been passed.