Charlie Tickler has been unceremoniously dumped with his sedentary, tv-obsessed grandparents in the village of Castle-on-the-Hudson, while his parents are off to Africa to save gorillas. Castle-on-the-Hudson may not have cell phone or Internet service, but it does have murder, intrigue, and a school for the deaf. Lonely and bored, Charlie decides to visit the library, where he meets an elderly deaf woman who looks frightened and tries to tell him something. But Charlie doesn't understand, and when it looks like the woman has disappeared, he searches for an answer. The answer comes by way of Frog-a local girl who is deaf. She reveals the woman's desperate message: dead. Charlie has no idea what this could mean, but Frog sees this as a chance to solve a real life mystery, just like her favorite fictional detective character, Dorrie McCann.

Now, Charlie must learn sign language to fully communicate with Frog, who despite being deaf interacts easily with everyone she meets. Charlie is a fast learner, and not only finds new ways to communicate his feelings, but also forges a lasting friendship.