Celine, Bobby Beaver and Oliver Owl investigate why an old library clock has begun producing one extra chime every hour. By recording a pattern, listening carefully to the different sounds and testing a prediction, they discover evidence that points to a problem inside the clock before a trained repairer confirms the cause. A thoughtful mystery story that teaches pattern recognition, careful listening, making and testing predictions, cause-and-effect reasoning, and solving problems safely without interfering with delicate equipment.