Derik becomes worried when his red bicycle disappears from the school rack, so Felix Fox and Milo Puppy help him investigate what happened. By examining a fallen notice, comparing tyre tracks, following the newest clues and checking identifying details, they discover that the bicycle had been moved to a safe storage area because of a damaged tree. A practical mystery story that teaches observation, evidence-based reasoning, staying calm, safety awareness, and checking the facts before reaching a conclusion.