Because of Timothy's reputation for telling stories, his teacher and classmates don't believe that a blue cat from the mountain has come through the school window to sleep in his desk. Later when the cat is adopted by the fourth grade class, his family doesn't believe his reports about Topcat living at school in a castle (which the children have made from cardboard cartons) and being checked out for weekends like a library book. On Timothy's weekend he houses Topcat in a tree because his mean landlady doesn't allow pets, but the cat disappears during a storm and Timothy goes in search of him to the misty mountain. After he is lost on the mountain, hurt in a fall, rescued by police and even written up in the newspaper, his story about meeting ""the spirit of the mountain"" is still not taken seriously -- until the bearded hermit who has indeed so introduced himself to Timothy comes down the mountain with Topcat, even visiting Timothy's class to straighten things out. So far the too familiar themes have been just about redeemed by the professionally inserted variations, but when in the last two end-tying pages the landlady relents about pets, Timothy announces that he's cured of fibbing ""forever and forever,"" and Dad answers that ""Dreams are all right as long as you know the difference,"" Topcat becomes the story that can't be believed.