Banana Fox's late fees have landed him in library jail! And when a familiar foe is found around town and a giant book-eating robot is unleashed, it looks like Flashlight and William need to step in to save the day. The junior detectives put their heads together, but when their investigation runs cold, an unlikely hero emerges and proves his own sleuthing skills! But without Banana Fox's help, can the trio put Sour Grapes Jr. behind bars for good?