Based on elements from the stories of Mark Twain, this feature-length Claymation fantasy follows the adventures of Tom Sawyer, Becky Thatcher and Huck Finn as they stowaway aboard the interplanetary balloon of Mark Twain. Twain, disgusted with the human race, is intent upon finding Halley's Comet and crashing into it, achieving his "destiny." It's up to Tom, Becky, and Huck to convince him that his judgment is wrong and that he still has much to offer humanity that might make a difference. Their efforts aren't just charitable; if they fail, they will share Twain's fate. Along the way, they use a magical time portal to get a detailed overview of the Twain philosophy, observing the "historical" events that inspired his works.

After a lightning bolt zaps a robot named Number 5, the lovable machine starts to think he's human and escapes the lab. Hot on his trail is his designer, Newton, who hopes to get to Number 5 before the military does. In the meantime, a spunky animal lover mistakes the robot for an alien and takes him in, teaching her new guest about life on Earth.