People are dying mysteriously and gruesomely, and nobody has a clue what the cause is. Only health worker Mike Brady has a possible solution, but his theory of killer slugs is laughed at by the authorities. Only when the body count begins to rise and a slug expert from England begins snooping around does it begin to look like Mike had the right idea after all.

A new member has joined Eli and the Shane Gang! Junjie, once the protector of the Eastern Caverns, is a master of the slugslinging art of Slug Fu! But even with the power of five slingers, the Shane Gang find themselves in over their heads as they race across The 99 Caverns in search of the Legendary Elemental Slugs. The five Elementals are ancient slugs of great power, and the forbearers of all slugs found in SlugTerra today. In the wrong hands, they could bring Slugterra to the brink of destruction. So when an evil alliance starts hunting down the Elementals, Eli and his friends — old and new — take off in pursuit of the greatest threat their world has ever faced!