Nelly and her dog London are about to spend autumn break with her uncle Hannibal. Soon she noticed that he doesn't live a quiet life. Her uncle is a Monster agent. Nelly gets dragged in to an adventure filled with ghosts, vampires and werewolves.

Nelly and the dog London are joined by Valle as they head out into the Black Forest to find the Mirror of Death. A journey that takes them deeper and deeper into the forest and to the place where Nelly's mother once disappeared.

When Nelly reunites with her mother, she learns that an ancient vampire intends to open the gateway to the underworld—where a slumbering evil waits to be unleashed. Together with Valle and London, they must stop the vampire before disaster strikes.