The graveyard riddle

Melody Bird is fascinated by the walled off "plague" house she discovered near the the graveyard, but she is even more fascinated by the mysterious boy, Hal, she finds living there in secret; he tells her he is a spy-in-training, a story that Melody's friends Matthew and Jake do not believe, and they set out to find out more--and uncover a riddle even more confusing than Hal's spy story.