Gappy is excited to travel with his best friend Pru to visit his cousins in the United Kingdom. His mother warns him to stay away from dangerous adventures, but after meeting two suspicious strangers in the sleepy Welsh village of Abergath, Gappy finds it difficult to obey. What are the two men up to, and what is the secret of the abandoned Penffordd Slate Mine? When Gappy and friends explore its underground tunnels, they uncover a shocking vampire plot before being captured and imprisoned in a rocky cell. Their only hope of escape is if Gappy can summon up a new vampire skill to rescue them all and lead everyone to safety.