Stag's army marches on, and up in the Sky fortress of Hackles, Mouse tries to keep hopeful as allies from different Tribes join forces to fight him. Mouse is forbidden to leave the mountain - but when their leader is kidnapped, she knows she must go in search of the last Opal, even if this means breaking her promise to Da ... Mouse is soon heading for the lands of the bloodthirsty Fangtooths. And as rumours fly about the return of an ancient evil, Mouse knows that she and her crew will soon face their biggest battle - for their Tribes, for their lives - for their world as they know it. The storm is coming ...