Nelly Rapp’s training at the Monster Academy has taught her that monsters and other dangerous beings can be managed with the help of the three C’s: calm, cleverness and cunning. But will they be enough to help her this time?

Nelly and her dog, London, are walking in the Troll Forest. There they come across an engineer who claims that the entire forest is to be cut down so that new buildings can be erected there, as long as the ground is stable enough. Then London suddenly vanishes in under a thick fir tree and Nelly must crawl in after him. There she comes eye-to-eye with none other than the Mountain King, the ancient being who rules the entire Troll Forest, but has no idea how to save it. Luckily, Nelly comes up with a plan: they must wake the giant of the forest. Because when he starts to move around after 900 years, he will literally rock the feet of the engineer up above…